How to kill hanging Windows remote desktop connection

When you don't log off Windows Remote Desktop (or terminal service client), the connection becomes
orphaned. You may get the following screen:

Too many sessions prevents future logins.  Do the following to clean up connections (on Windows 2003 server).
From a client machines CMD windows:

D:>query session /SERVER:myserver.myco.com
SESSIONNAME       USERNAME                 ID  STATE   TYPE        DEVICE
console                                     0  Conn    wdcon
rdp-tcp                                 65536  Listen  rdpwd
rdp-tcp#11        Administrator             1  Active  rdpwd
rdp-tcp#4         Administrator             2  Active  rdpwd

The above show 2 connections to “myserver”. You can kill the sessions with ID 1, 2, 3, so on.

tsdiscon 1 /SERVER:myserver.myco.com

where 1  rdp-tcp#11        Administrator             1  Active  rdpwd “ID” col
Also, can tskill to remotely stop a process.

Scheduling Linux Tasks

automation and scheduling:

The at command enables the simplest form of job scheduling. It runs a command or a series of commands once only at a specified time, and mails any output to the user. The jobs it runs are processed in the background. You use this syntax for the at command. In the syntax, you can specify the time in a number of different ways. For example, you can specify the time as 01:30 or as 1:30 am.

[root@host11 pwstaff]# at -m 01:30


You now want to schedule a series of commands that you’ve saved in a file named “system_maintenance” to run at 12:00 pm. To do this, you use the -f option to Specify the file in an at command.

[root@host11 pwstaff]# at -f system_maintenance 12:00

Suppose that it is the end of June and you want to schedule a job to run four weeks from today, without entering the exact date on which it must run. Which command do you enter before specifying the job that must run to do this?

at now + 4 weeks

Ordinary users use the crontab command to create or edit their crontab files. To use the crontab command, users must be listed in the file /etc/cron.allow. If that file doesn’t exist, the users must not be listed in /etc/cron.deny. If neither file exists, all users can use the crontab command unless some other configuration prevents this.

[root@samwise root]# crontab -l

no crontab for root

[root@samwise root]# crontab -e to create

An entry of 00 in the minute field, 20 in the hour field, and 1-5 in the day of week field specifies that a command must run at 8:00 pm every day, from Monday to Friday.

00 20 * * 1-5 sort/usr/sales


In order, the fields that make up the time pattern in a crontab file entry include minute, hour, day of month, month of year, and day of week. Examples:


0 22 * * 1-5 10:00 pm on weekdays
15 14 1 * * 2:15 pm on the first of every month
30 /2 * * * Half past the hour every two hours
30 19 * * 1 7:30 pm every Monday


The /etc/at.allow and /etc/at.deny files enable you to control which users can run the at command. When a user tries to run an at job, the at command checks to see if the /etc/at.allow file exists. If it does and the user is listed in the file, at schedules the job.


If the user is not listed in /etc/at.allow, they will not be able to schedule the job, irrespective of whether they are listed in the /etc/at.deny file.If the /etc/at.allow file does not exist, at checks the /etc/at.deny file. If the user’s name does not appear in this file, the job will be scheduled. If the /etc/at.allow file doesn’t exist and /etc/at.deny is empty, any user can run at jobs. Alternatively, if /etc/at.allow exists and is empty, or if neither file exists, no user except root can run at jobs.

In some Linux distributions, such as Red Hat 7, any user can run cron jobs if neither the /etc/cron.allow or /etc/cron.deny files exist. In other distributions, only root can run cron jobs if neither file exists.

How to export NFS share and mount it on a client (AIX)

bash-3.00# exportfs
/data/fs/fs1 -rw=pw41,access=pw41.test.pw.com,root=pw41.test.pw.com

NOTE: the exported directory (e.g., /data/fs/fs1) must be a file system itself, not just any directory.
In this case, /data/fs/fs1 is a file system itself, but /data is not. So if you export /data, when you
mount it from client, it will not work (you will get an error).

bash-3.00# exportfs -a

This commands exports (or freshes) the list again from the server. Man page for exportfs shows this:
Flags

-a Exports all directories listed in the exports file.
-v Prints the name of each directory as it is exported or unexported.

To verify what’s been exported, run:
bash-3.00# exportfs
/cesa/fs/fs1 -rw=pw41,access=pw41.test.pw.com,root=pw41.test.pw.com

From client side, you can run showmount to list what’s exported:
bash-3.00# showmount -a pw40
pw41.test.pw.com:/data/fs/fs1

Then you mount it to an existing directory (create it if it doesn’t exist yet):

bash-3.00# mount pw40:/data/fs/fs1 /data/clientfs
bash-3.00# cd /data/clientfs
bash-3.00# ls

DB2 V9.1: How to Re-create a Instance

Like the Nissan Pathfinder’s ad: “not that you would, but you could”. In certain performance test conditions, you may have to drop the current DB2 V8 instance, install V9.1, and re-create a instance. Here is a short list of helpful commands - run as root unless specifically sudo’ed:

1. Drop DB2 instance

/opt/IBM/db2/V9.1/instance/db2idrop -f db2inst1
OR version 8
/usr/opt/db2_08_01/instance/db2idrop -f db2inst1

2. Uninstall previous db2

Stop and drop the DB2 Admin server first.

bash-3.00# su dasusr
bash-3.00$ pwd
/home/dasusr/das/bin
bash-3.00$ ./db2admin stop
SQL4407W The DB2 Administration Server was stopped successfully.

Drop the DAS server as root:

bash-3.00# pwd
/opt/IBM/db2/V9.1/instance
bash-3.00# ./dasdrop
SQL4410W The DB2 Administration Server is not active.
DBI1070I Program dasdrop completed successfully.

Then db2_deinstall UDB:

bash-3.00# pwd
/opt/IBM/db2/V9.1/install
bash-3.00# ./db2_deinstall -a
DBI1016I Program db2_deinstall is performing uninstallation.
Please wait.
The execution completed successfully.

3. Run db2 V9.1 install, db2_install will install at console (non-GUI).

4. After install, create an instance

Make sure db2fence1 and db2inst1 directories created under /home and with correct
permissions:
drwxr-xr-x 2 db2fenc1 db2admi 256 May 04 15:19 db2fenc1
drwxr-xr-x 2 db2inst1 db2admi 256 May 04 15:19 db2inst1
then

/opt/IBM/db2/V9.1/instance/db2icrt -u db2fenc1 db2inst1

bash-3.00# ./db2icrt -u db2fenc1 db2inst1
DBI1070I Program db2icrt completed successfully.

ORA-12705: Cannot access NLS data files or invalid environment specified

This tip resolves Oracle client error: “ORA-12705: Cannot access NLS data files or invalid environment specified“. According to Oracle documentation, “ORA-12705″ can happen under the following conditions:

1. ORA-12705 with incorrect NLS_LANG parameter
2. ORA-12705 with incorrectly specified ORA_NLSx
3. ORA-12705 from incorrect Oracle install or configuration
4. ORA-12705 when using Special Character sets
5. ORA-12705 when connecting with SQL*Net
6. ORA-12705 during migrations
7. ORA-12705 when connecting with SQL*Plus
8. ORA-12705 when connecting with language pre-compilers
9. ORA-12705 during Export/Import

Try doing the following to fix this error on the client side. On the client Windows machine (this tip
is only for Windows), The NLS_LANG must be unset in the Windows registry (re-naming works the best because you know what you changed). Look for the NLS_LANG subkey in the registry at \HKEY_LOCAL_MACHINE\SOFTWARE\ORACLE, and rename it. Then try SQLPlus again.

How to "tar" Only Directories, Not Files

Sometimes you want to preserve only a directory structure into a tar ball without having to archive the files themselves. This can happen if you want to backup a pre-defined directory structure, run some tests, and do not really care about the files created during the test. You can use the UNIX commands ‘find’ and ‘tar’ to do that.

# find bwtest -type d > dirlist.txt
# more dirlist.txt
bwtest
bwtest/dd1
bwtest/dd1/1
bwtest/dd1/458986
bwtest/dd1/217128
bwtest/dd1/217136
bwtest/dd1/217182
bwtest/dd2
bwtest/dd2/2
bwtest/dd3
bwtest/dd3/3
bwtest/reports
# tar -cvf ./bwtest.tar -L dirlist.txt

content.tar will contain only the directories, no files. This should work on
all UNIX and Linux platforms.

Simple Commands To Backup and Restore a Oracle Database

Below shows simple steps in Oracle 9i and 10g to backup and restore Oracle databases; the goal is for the tasks to be called remotely so they can be automated as part of a performance test execution (i.e., always restores a known baseline before start a test!) Applies to Windows (steps apply to other platform with minor modifications).

1) Shuts down the listener:

F:\_backup>lsnrctl stop

LSNRCTL for 32-bit Windows: Version 10.2.0.3.0 - Production on 23-MAY-2007 09:49:20

Copyright (c) 1991, 2006, Oracle. All rights reserved.

Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=pw213)(PORT=1521)))
The command completed successfully

2) Shuts down the instance:

F:\_backup>sqlplus /nolog

SQL*Plus: Release 10.2.0.3.0 - Production on Wed May 23 09:49:27
2007

Copyright (c) 1982, 2006, Oracle. All Rights Reserved.

SQL> connect sys as sysdba
Enter password:
Connected.
SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> quit
Disconnected from Oracle Database 10g Enterprise Edition Release 10.2.0.3.0 - Production
With the Partitioning, OLAP and Data Mining options

3) Use operating system “copy” command to copy the entire “oradata” directory to your backup location:

F:\_backup>xcopy /Y/E D:\oradata oradata_2007_05_21

Copies all oradata to the
“oradata_2007_05_21″ directory.

4) To restore, repeat steps 1) and 2), then copy back the files from “oradata_2007_05_21″ to your run-time “oradata” directory.

5) Start up listener:

F:\_backup>lsnrctl start

6) Start up instance:

F:\_backup>sqlplus /nolog

SQL*Plus: Release 10.2.0.3.0 - Production on Wed May 23 09:49:27 2007

Copyright (c) 1982, 2006, Oracle. All Rights Reserved.

SQL> connect sys as sysdba
Enter password:
Connected to an idle instance.
SQL> startup

5 Steps To Increase Storage for Your Existing Logical Volume (AIX)

Steps taken to to increase the storage for an existing logical volume. The following commands
are for AIX. The steps, however, are universal. You only need to substitute the commands for other platforms.

1) How big is the vg?

/home/oracle $ lsvg oradata_vg
VOLUME GROUP: oradata_vg VG IDENTIFIER: 00224ada00004c000000010f64352701
VG STATE: active PP SIZE: 128 megabyte(s)
VG PERMISSION: read/write TOTAL
PPs: 1086 (139008 megabytes)
MAX LVs: 256 FREE PPs: 685 (87680 megabytes)
LVs: 2 USED PPs: 401 (51328 megabytes)
OPEN LVs: 2 QUORUM: 2
TOTAL PVs:
2 VG DESCRIPTORS: 3
STALE PVs: 0 STALE PPs: 0
ACTIVE PVs: 2 AUTO ON: yes
MAX PPs per VG: 32512
MAX PPs per PV: 1016 MAX PVs: 32
LTG size (Dynamic): 128
kilobyte(s) AUTO SYNC: no
HOT SPARE: no BB POLICY: relocatable

2) Use lspv to find out which hdisk belong to which vg:

/home/oracle $ lspv
hdisk2 00224ada0abc41b2 rootvg active
hdisk0 00224adaee4d5b0e None
hdisk1 00224adaa0d19b45 None
hdisk3 00224ada432ae745 oradata_vg active
hdisk4 00224adaf645417c oradata_vg active
hdisk5 00224adafc227f17 udbdata_vg active
hdisk6 00224adafc22d1ed udbdata_vg active
hdisk7 00224adafc3ab232 udbdata_vg active
hdisk8 00224adafc3ab64b udbdata_vg active
hdisk9 0003321524243690 udbdata_vg active
hdisk10 00033215242458d4 udbdata_vg active
hdisk11 00224adab4c1999c pagevg active
hdisk12 00224adab4c199c9 oracle_vg active
hdisk13 00224adafc20bbc5 oracle_vg active
hdisk14 00224adafc20bed6 oracle_vg active

hdisk3 and hdisk4 belong to oradata_vg.

3) how large is each hdisk?

/home/oracle $ lspv hdisk3
PHYSICAL VOLUME: hdisk3 VOLUME GROUP: oradata_vg
PV IDENTIFIER: 00224ada432ae745 VG IDENTIFIER 00224ada00004c000000010f64352701
PV STATE: active
STALE PARTITIONS:
0 ALLOCATABLE: yes
PP SIZE: 128 megabyte(s) LOGICAL VOLUMES: 2
TOTAL PPs: 543 (69504 megabytes) VG DESCRIPTORS: 2
FREE PPs: 342 (43776 megabytes) HOT SPARE: no
USED PPs: 201 (25728 megabytes) MAX REQUEST: 128 kilobytes
FREE DISTRIBUTION: 108..00..17..108..109
USED DISTRIBUTION: 01..109..91..00..00
/home/oracle $
lspv hdisk4
PHYSICAL VOLUME: hdisk4 VOLUME GROUP: oradata_vg
PV IDENTIFIER: 00224adaf645417c VG IDENTIFIER 00224ada00004c000000010f64352701
PV STATE: active
STALE PARTITIONS: 0 ALLOCATABLE: yes
PP SIZE: 128 megabyte(s) LOGICAL VOLUMES: 1
TOTAL PPs: 543 (69504 megabytes) VG DESCRIPTORS: 1
FREE
PPs: 343 (43904 megabytes) HOT SPARE: no
USED PPs: 200 (25600 megabytes) MAX REQUEST: 128 kilobytes
FREE DISTRIBUTION: 109..00..17..108..109
USED DISTRIBUTION: 00..109..91..00..00

so total about 69G _ 69G = 140G.

4) now, how large is the oradata_lv currently?

/home/oracle $ lslv oradata_lv
LOGICAL VOLUME: oradata_lv VOLUME GROUP: oradata_vg
LV IDENTIFIER: 00224ada00004c000000010f64352701.1 PERMISSION: read/write
VG
STATE: active/complete LV STATE: opened/syncd
TYPE: jfs2 WRITE VERIFY: off
MAX LPs: 512 PP SIZE: 128 megabyte(s)
COPIES: 1 SCHED POLICY:
striped
LPs: 400 PPs: 400
STALE PPs: 0 BB POLICY: relocatable
INTER-POLICY: maximum RELOCATABLE: no
INTRA-POLICY: middle UPPER BOUND: 2
MOUNT POINT:
/oradata LABEL: /oradata
MIRROR WRITE CONSISTENCY: on/ACTIVE
EACH LP COPY ON A SEPARATE PV ?: yes (superstrict)
Serialize IO ?: NO
STRIPE WIDTH: 2
STRIPE SIZE: 128K

looks like LPs=400 each is 128M = 51GB.
Still available 140G - 51GB = 90GB.

and from the lsvg, we know that:

VG PERMISSION: read/write TOTAL PPs: 1086 (139008 megabytes)
MAX LVs: 256 FREE
PPs: 685 (87680 megabytes)
LVs: 2 USED PPs: 401 (51328 megabytes)

5a) unmount the fs first
5b) change the logical volume in smitty - increase MAXIMUM NUMBER OF LOGICAL PARTITIONS to 1086 - ran OK
5c) increase size of lv, enter number of ADDITIONAL LPs: to 684 (not 685 because stripped volumes require even numbers; also don’t choose PVs) - ran OK
5d) change file system, Change/Show characteristics of an Enhanced JFS; set the new
“number of units (of 512)” to 283978437 - ran OK
5e) check the new fs:

6) Validate everything:

bash-2.05b# df -k
Filesystem 1024-blocks Free %Used Iused %Iused Mounted on
/dev/oradata_lv 142082048 106387720 26% 22 1% /oradata

the logical volume is now 142GB! It worked.

A Picture of the typical IO Stack

I/O stack components Overview

Typically the stack consists of the following
• Database
• Character or block device driver
• Multipathing driver
• SCSI driver
• HBA
• System Bus
• Switch/iSCSI routers
• Storage Array
• Disks

Example of vendors:

ASM-Database IBM Oracle
Character or block device driver
Multipathing driver – PowerPath, Windows MPIO
SCSI driver
HBA – Host Bus Adapter, Qlogic (IBM), Emulex
System Bus - PCI
Switch/iSCSI routers – Brocade, McData
Storage Array – IBM DS8100, EMC, HDS, HP
Disks - Seagate

The database processes makes I/O calls to the block device. The I/O scheduler in Linux forms the
interface between the generic block layer and the low level device drivers. The block layer provides
functions that are utilized by the file systems, databases (10gR2) and the virtual memory manager to submit I/O requests to block devices. These requests are transformed by the I/O scheduler and made available to the low-level device drivers. The device drivers consume the transformed requests and forward them (by using device specific protocols) to the actual device controllers that perform the I/O operations.

IOPS vs. Mbytes/sec

IOPS

• The standard unit of measurement for I/O operations per second. Includes all reads and writes.
• This is how you rate a I/O stack components ability to process small block random I/O requests.
• Used to describe I/O rate driven applications (OLTP, random I/O ) Mbytes/s


Mega”Bytes” per sec

• Used to measure large block sequential transfer rates, with no response time characterization
• Used to describe data rate driven applications (DSS, OLAP)
• Transaction/s – its anything you claim it to be.

Oracle query to gather this data from your database:

set lines 250
set pages 9999
spool sysmetric_outp.log
alter session set nls_date_format=’dd-mm-yyyy hh24:mi’;
select min(begin_time), max(end_time),
sum(case metric_name when ‘Physical Read Total Bytes Per Sec’ then average end)
Physical_Read_Total_Bps,
sum(case metric_name when ‘Physical Write Total Bytes Per Sec’ then average end)
Physical_Write_Total_Bps,
sum(case metric_name when ‘Redo Generated Per Sec’ then average end)
Redo_Bytes_per_sec,
sum(case metric_name when ‘Physical Read Total IO Requests Per Sec’ then average end)
Physical_Read_IOPS,
sum(case metric_name when ‘Physical Write Total IO Requests Per Sec’ then average end)
Physical_write_IOPS,
sum(case metric_name when ‘Redo Writes Per Sec’ then average end) Physical_redo_IOPS,
sum(case metric_name when ‘Current OS Load’ then average end) OS_LOad,
sum(case metric_name when ‘CPU Usage Per Sec’ then average end)
DB_CPU_Usage_per_sec,
sum(case metric_name when ‘Host CPU Utilization (%)’ then average end) Host_CPU_util, –
NOTE 100% = 1 loaded RAC node
sum(case metric_name when ‘Network Traffic Volume Per Sec’ then average end)
Network_bytes_per_sec,
snap_id
from dba_hist_sysmetric_summary
group by snap_id
order by snap_id;
spool off

[source: CERN]

How Much IO Does Your Application Generate?

IO rate of 1+ Gigabytes/sec is rarely achievable, but hundreds of Megabytes/sec is achieved in lots of high-end production environment. Keep those numbers in mind when planning for what’s possible.

In Oracle, use AWR reports to determine current I/O metrics (Instance Activity Stats per sec).

• IOPS = “physical reads total I/O requests” + “physical writes total I/O requests”
• MBytes/s = “physical reads total bytes” + physical writes total bytes”
• For RAC environments - aggregate IOPS or MBytes/s for all nodes

Now the tricky part – estimating expected I/O throughput rates

• Establish what is being expanded – more users, new reports
• Pull I/O stats for a given session or transaction and extrapolate to the required number of users.
• Ensure data points are representative of the new workload
• Know when your Web users get on your system (e.g., peak hour transactions)

For new or non-existing applications, use business rules or data model transaction profiles flow to understand “what is a transaction”, and then extrapolate for transactions/s or per hour. Optionally you can use the numbers we have seen in our consulting gigs. Note that these are just guideline values.
Use the following as basic guidelines for OLTP (Online transaction processing) systems :

Low transaction system – 1000 IOPS or 200MBytes/s
Medium transaction system – 5000 IOPS or 600 Mbytes/s
High-end transaction system – 10,000 IOPS or 1Gbytes/s (almost rarely achievable and usually TPC-C type workloads)

Use the following as basic guidelines for DSS (Decision Support Systems) (units are in Gig = Gigabytes/Sec):

2 Gig I/O entry level DW
4-8 Gig I/O High End DW
10+ Gig I/O Very High END DW not yet seen in production !
40+ Gig I/O High End TPC-H

Common Linux Commands By Examples

These are usually invoked by root and used for system maintenance or emergency filesystem repairs. Use with caution, as some of these commands may damage your system if misused.

Users and Groups

users

Show all logged on users. This is the approximate equivalent of who -q.

groups

Lists the current user and the groups she belongs to. This corresponds to the $GROUPS internal variable, but gives the group names, rather than the numbers.

bash$ groups bozita cdrom cdwriter audio xgrp bash$ echo $GROUPS 501

chown, chgrp

The chown command changes the ownership of a file or files. This command is a useful method that root can use to shift file ownership from one user to another. An ordinary user may not change the ownership of files, not even her own files.

root# chown bozo *.txt

The chgrp command changes the group ownership of a file or files. You must be owner of the file(s) as well as a member of the destination group (or root) to use this operation.

chgrp –recursive dunderheads *.data # The “dunderheads” group will now own all the “*.data” files #+ all the way down the $PWD directory tree (that’s what “recursive” means).

useradd, userdel

The useradd administrative command adds a user account to the system and creates a home directory for that particular user, if so specified. The corresponding userdel command removes a user account from the system and deletes associated files.

NOTE	The adduser command is a synonym for useradd and is usually a symbolic link to it.

id

The id command lists the real and effective user IDs and the group IDs of the current user. This is the counterpart to the $UID, $EUID, and $GROUPS internal Bash variables.

bash$ id uid=501(bozo) gid=501(bozo) groups=501(bozo),22(cdrom),80(cdwriter),81(audio) bash$ echo $UID 501

who

Show all users logged on to the system.

bash$ who bozo tty1 Apr 27 17:45 bozo pts/0 Apr 27 17:46 bozo pts/1 Apr 27 17:47 bozo pts/2 Apr 27 17:49

The -m gives detailed information about only the current user. Passing any two arguments to who is the equivalent of who -m, as in who am i or who The Man.

bash$ who -m localhost.localdomain!bozo pts/2 Apr 27 17:49

whoami is similar to who -m, but only lists the user name.

bash$ whoami bozo

w

Show all logged on users and the processes belonging to them. This is an extended version of who. The output of w may be piped to grep to find a specific user and/or process.

bash$ w | grep startx bozo tty1 - 4:22pm 6:41 4.47s 0.45s startx

logname

Show current user’s login name (as found in /var/run/utmp). This is a near-equivalent to whoami, above.

bash$ logname bozo bash$ whoami bozo

However…

bash$ su Password: …… bash# whoami root bash# logname bozo

su

Runs a program or script as a substitute user. su rjones starts a shell as user rjones. A naked su defaults to root.

sudo

Runs a command as root (or another user). This may be used in a script, thus permitting a regular user to run the script.

#!/bin/bash # Some commands. sudo cp /root/secretfile /home/bozo/secret # Some more commands.

The file /etc/sudoers holds the names of users permitted to invoke sudo.

passwd

Sets or changes a user’s password.

The passwd can be used in a script, but should not be.

#!/bin/bash # set-new-password.sh: Not a good idea. # This script must be run as root, #+ or better yet, not run at all. ROOT_UID=0 # Root has $UID 0. E_WRONG_USER=65 # Not root? if [ “$UID” -ne “$ROOT_UID” ] then echo; echo “Only root can run this script.”; echo exit $E_WRONG_USER else echo; echo “You should know better than to run this script, root.” fi username=bozo NEWPASSWORD=security_violation echo “$NEWPASSWORD” | passwd –stdin “$username” # The ‘–stdin’ option to ‘passwd’ permits #+ getting new password from stdin (or a pipe). echo; echo “User $username’s password changed!” # Using the ‘passwd’ command in a script is dangerous. exit 0

ac

Show users’ logged in time, as read from /var/log/wtmp. This is one of the GNU accounting utilities.

bash$ ac total 68.08

last

List last logged in users, as read from /var/log/wtmp. This command can also show remote logins.

newgrp

Change user’s group ID without logging out. This permits access to the new group’s files. Since users may be members of multiple groups simultaneously, this command finds little use.

Terminals

tty

Echoes the name of the current user’s terminal. Note that each separate xterm window counts as a different terminal.

bash$ tty /dev/pts/1

stty

Shows and/or changes terminal settings. This complex command, used in a script, can control terminal behavior and the way output displays. See the info page, and study it carefully.

Example 13-1. setting an erase character

#!/bin/bash # erase.sh: Using “stty” to set an erase character when reading input. echo -n “What is your name? “ read name # Try to erase characters of input. # Won’t work. echo “Your name is $name.” stty erase ‘#’ # Set “hashmark” (#) as erase character. echo -n “What is your name? “ read name # Use # to erase last character typed. echo “Your name is $name.” exit 0

Example 13-2. secret password: Turning off terminal echoing

#!/bin/bash echo echo -n “Enter password “ read passwd echo “password is $passwd” echo -n “If someone had been looking over your shoulder, “ echo “your password would have been compromised.” echo && echo # Two line-feeds in an “and list”. stty -echo # Turns off screen echo. echo -n “Enter password again “ read passwd echo echo “password is $passwd” echo stty echo # Restores screen echo. exit 0

A creative use of stty is detecting a user keypress (without hitting ENTER).

Example 13-3. Keypress detection

#!/bin/bash # keypress.sh: Detect a user keypress (”hot keyboard”). echo old_tty_settings=$(stty -g) # Save old settings. stty -icanon Keypress=$(head -c1) # or $(dd bs=1 count=1 2> /dev/null) # on non-GNU systems echo echo “Key pressed was ”"$Keypress””.” echo stty “$old_tty_settings” # Restore old settings. # Thanks, Stephane Chazelas. exit 0

terminals and modes Normally, a terminal works in the canonical mode. When a user hits a key, the resulting character does not immediately go to the program actually running in this terminal. A buffer local to the terminal stores keystrokes. When the user hits the ENTER key, this sends all the stored keystrokes to the program running. There is even a basic line editor inside the terminal. bash$ stty -a speed 9600 baud; rows 36; columns 96; line = 0; intr = ^C; quit = ^; erase = ^H; kill = ^U; eof = ^D; eol = ; eol2 = ; start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R; werase = ^W; lnext = ^V; flush = ^O; … isig icanon iexten echo echoe echok -echonl -noflsh -xcase -tostop -echoprt Using canonical mode, it is possible to redefine the special keys for the local terminal line editor. bash$ cat > filexxx whaIfoo barhello world bash$ cat filexxx hello world bash$ bash$ wc -c <> 13 The process controlling the terminal receives only 13 characters (12 alphabetic ones, plus a newline), although the user hit 26 keys. In non-canonical (“raw”) mode, every key hit (including special editing keys such as ctl-H) sends a character immediately to the controlling process. The Bash prompt disables both icanon and echo, since it replaces the basic terminal line editor with its own more elaborate one. For example, when you hit ctl-A at the Bash prompt, there’s no ^A echoed by the terminal, but Bash gets a \1 character, interprets it, and moves the cursor to the begining of the line.

tset

Show or initialize terminal settings. This is a less capable version of stty.

bash$ tset -r Terminal type is xterm-xfree86. Kill is control-U (^U). Interrupt is control-C (^C).

setserial

Set or display serial port parameters. This command must be run by root user and is usually found in a system setup script.

# From /etc/pcmcia/serial script: IRQ=`setserial /dev/$DEVICE | sed -e ’s/.*IRQ: //’` setserial /dev/$DEVICE irq 0 ; setserial /dev/$DEVICE irq $IRQ

getty, agetty

The initialization process for a terminal uses getty or agetty to set it up for login by a user. These commands are not used within user shell scripts. Their scripting counterpart is stty.

mesg

Enables or disables write access to the current user’s terminal. Disabling access would prevent another user on the network to write to the terminal.

TIP	It can be very annoying to have a message about ordering pizza suddenly appear in the middle of the text file you are editing. On a multi-user network, you might therefore wish to disable write access to your terminal when you need to avoid interruptions.

wall

This is an acronym for “write all”, i.e., sending a message to all users at every terminal logged into the network. It is primarily a system administrator’s tool, useful, for example, when warning everyone that the system will shortly go down due to a problem (see Example 17-2).

bash$ wall System going down for maintenance in 5 minutes! Broadcast message from bozo (pts/1) Sun Jul 8 13:53:27 2001… System going down for maintenance in 5 minutes!

NOTE	If write access to a particular terminal has been disabled with mesg, then wall cannot send a message to it.

dmesg

Lists all system bootup messages to stdout. Handy for debugging and ascertaining which device drivers were installed and which system interrupts in use. The output of dmesg may, of course, be parsed with grep, sed, or awk from within a script.

bash$ dmesg | grep hda Kernel command line: ro root=/dev/hda2 hda: IBM-DLGA-23080, ATA DISK drive hda: 6015744 sectors (3080 MB) w/96KiB Cache, CHS=746/128/63 hda: hda1 hda2 hda3 <> hda4

Information and Statistics

uname

Output system specifications (OS, kernel version, etc.) to stdout. Invoked with the -a option, gives verbose system info (see Example 12-4). The -s option shows only the OS type.

bash$ uname -a Linux localhost.localdomain 2.2.15-2.5.0 #1 Sat Feb 5 00:13:43 EST 2000 i686 unknown bash$ uname -s Linux

arch

Show system architecture. Equivalent to uname -m. See Example 10-25.

bash$ arch i686 bash$ uname -m i686

lastcomm

Gives information about previous commands, as stored in the /var/account/pacct file. Command name and user name can be specified by options. This is one of the GNU accounting utilities.

lastlog

List the last login time of all system users. This references the /var/log/lastlog file.

bash$ lastlog root tty1 Fri Dec 7 18:43:21 -0700 2001 bin **Never logged in** daemon **Never logged in** … bozo tty1 Sat Dec 8 21:14:29 -0700 2001 bash$ lastlog | grep root root tty1 Fri Dec 7 18:43:21 -0700 2001

CAUTION

This command will fail if the user invoking it does not have read permission for the /var/log/lastlog file.

lsof

List open files. This command outputs a detailed table of all currently open files and gives information about their owner, size, the processes associated with them, and more. Of course, lsof may be piped to grep and/or awk to parse and analyze its results.

bash$ lsof COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME init 1 root mem REG 3,5 30748 30303 /sbin/init init 1 root mem REG 3,5 73120 8069 /lib/ld-2.1.3.so init 1 root mem REG 3,5 931668 8075 /lib/libc-2.1.3.so cardmgr 213 root mem REG 3,5 36956 30357 /sbin/cardmgr …

strace

Diagnostic and debugging tool for tracing system calls and signals. The simplest way of invoking it is strace COMMAND.

bash$ strace df execve(”/bin/df”, [”df”], [/* 45 vars */]) = 0 uname({sys=”Linux”, node=”bozo.localdomain”, …}) = 0 brk(0) = 0×804f5e4 …

This is the Linux equivalent of truss.

free

Shows memory and cache usage in tabular form. The output of this command lends itself to parsing, using grep, awk or Perl. The procinfo command shows all the information that free does, and much more.

bash$ free total used free shared buffers cached Mem: 30504 28624 1880 15820 1608 16376 -/+ buffers/cache: 10640 19864 Swap: 68540 3128 65412

To show unused RAM memory:

bash$ free | grep Mem | awk ‘{ print $4 }’ 1880

procinfo

Extract and list information and statistics from the /proc pseudo-filesystem. This gives a very extensive and detailed listing.

bash$ procinfo | grep Bootup Bootup: Wed Mar 21 15:15:50 2001 Load average: 0.04 0.21 0.34 3/47 6829

lsdev

List devices, that is, show installed hardware.

bash$ lsdev Device DMA IRQ I/O Ports ———————————————— cascade 4 2 dma 0080-008f dma1 0000-001f dma2 00c0-00df fpu 00f0-00ff ide0 14 01f0-01f7 03f6-03f6 …

du

Show (disk) file usage, recursively. Defaults to current working directory, unless otherwise specified.

bash$ du -ach 1.0k ./wi.sh 1.0k ./tst.sh 1.0k ./random.file 6.0k . 6.0k total

df

Shows filesystem usage in tabular form.

bash$ df Filesystem 1k-blocks Used Available Use% Mounted on /dev/hda5 273262 92607 166547 36% / /dev/hda8 222525 123951 87085 59% /home /dev/hda7 1408796 1075744 261488 80% /usr

stat

Gives detailed and verbose statistics on a given file (even a directory or device file) or set of files.

bash$ stat test.cru File: “test.cru” Size: 49970 Allocated Blocks: 100 Filetype: Regular File Mode: (0664/-rw-rw-r–) Uid: ( 501/ bozo) Gid: ( 501/ bozo) Device: 3,8 Inode: 18185 Links: 1 Access: Sat Jun 2 16:40:24 2001 Modify: Sat Jun 2 16:40:24 2001 Change: Sat Jun 2 16:40:24 2001

If the target file does not exist, stat returns an error message.

bash$ stat nonexistent-file nonexistent-file: No such file or directory

vmstat

Display virtual memory statistics.

bash$ vmstat procs memory swap io system cpu r b w swpd free buff cache si so bi bo in cs us sy id 0 0 0 0 11040 2636 38952 0 0 33 7 271 88 8 3 89

netstat

Show current network statistics and information, such as routing tables and active connections. This utility accesses information in /proc/net.

netstat -r is equivalent to route.

uptime

Shows how long the system has been running, along with associated statistics.

bash$ uptime 10:28pm up 1:57, 3 users, load average: 0.17, 0.34, 0.27

hostname

Lists the system’s host name. This command sets the host name in an /etc/rc.d setup script (/etc/rc.d/rc.sysinit or similar). It is equivalent to uname -n, and a counterpart to the $HOSTNAME internal variable.

bash$ hostname localhost.localdomain bash$ echo $HOSTNAME localhost.localdomain

hostid

Echo a 32-bit hexadecimal numerical identifier for the host machine.

bash$ hostid 7f0100

NOTE

This command allegedly fetches a “unique” serial number for a particular system. Certain product registration procedures use this number to brand a particular user license. Unfortunately, hostid only returns the machine network address in hexadecimal, with pairs of bytes transposed.The network address of a typical non-networked Linux machine, is found in /etc/hosts. bash$ cat /etc/hosts 127.0.0.1 localhost.localdomain localhost As it happens, transposing the bytes of 127.0.0.1, we get 0.127.1.0, which translates in hex to 007f0100, the exact equivalent of what hostid returns, above. There exist only a few million other Linux machines with this identical hostid.

sar

Invoking sar (system activity report) gives a very detailed rundown on system statistics. This command is found on some commercial UNIX systems, but is not part of the base Linux distribution. It is contained in the sysstat utilities package, written by Sebastien Godard.

bash$ sar Linux 2.4.7-10 (localhost.localdomain) 12/31/2001 10:30:01 AM CPU %user %nice %system %idle 10:40:00 AM all 1.39 0.00 0.77 97.84 10:50:00 AM all 76.83 0.00 1.45 21.72 11:00:00 AM all 1.32 0.00 0.69 97.99 11:10:00 AM all 1.17 0.00 0.30 98.53 11:20:00 AM all 0.51 0.00 0.30 99.19 06:30:00 PM all 100.00 0.00 100.01 0.00 Average: all 1.39 0.00 0.66 97.95

readelf

Show information and statistics about a designated elf binary. This is part of the binutils package.

bash$ readelf -h /bin/bash ELF Header: Magic: 7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00 Class: ELF32 Data: 2’s complement, little endian Version: 1 (current) OS/ABI: UNIX - System V ABI Version: 0 Type: EXEC (Executable file) . . .

size

The size [/path/to/binary] command gives the segment sizes of a binary executable or archive file. This is mainly of use to programmers.

bash$ size /bin/bash text data bss dec hex filename 495971 22496 17392 535859 82d33 /bin/bash

System Logs

logger

Appends a user-generated message to the system log (/var/log/messages). You do not have to be root to invoke logger.

logger Experiencing instability in network connection at 23:10, 05/21. # Now, do a ‘tail /var/log/messages’.

By embedding a logger command in a script, it is possible to write debugging information to /var/log/messages.

logger -t $0 -i Logging at line “$LINENO”. # The “-t” option specifies the tag for the logger entry. # The “-i” option records the process ID. # tail /var/log/message # … # Jul 7 20:48:58 localhost ./test.sh[1712]: Logging at line 3.

logrotate

This utility manages the system log files, rotating, compressing, deleting, and/or mailing them, as appropriate. Usually crond runs logrotate on a daily basis.

Adding an appropriate entry to /etc/logrotate.conf makes it possible to manage personal log files, as well as system-wide ones.

Job Control

ps

Process Statistics: lists currently executing processes by owner and PID (process id). This is usually invoked with ax options, and may be piped to grep or sed to search for a specific process.

bash$ ps ax | grep sendmail 295 ? S 0:00 sendmail: accepting connections on port 25

pstree

Lists currently executing processes in “tree” format. The -p option shows the PIDs, as well as the process names.

top

Continuously updated display of most cpu-intensive processes. The -b option displays in text mode, so that the output may be parsed or accessed from a script.

bash$ top -b 8:30pm up 3 min, 3 users, load average: 0.49, 0.32, 0.13 45 processes: 44 sleeping, 1 running, 0 zombie, 0 stopped CPU states: 13.6% user, 7.3% system, 0.0% nice, 78.9% idle Mem: 78396K av, 65468K used, 12928K free, 0K shrd, 2352K buff Swap: 157208K av, 0K used, 157208K free 37244K cached PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND 848 bozo 17 0 996 996 800 R 5.6 1.2 0:00 top 1 root 8 0 512 512 444 S 0.0 0.6 0:04 init 2 root 9 0 0 0 0 SW 0.0 0.0 0:00 keventd …

nice

Run a background job with an altered priority. Priorities run from 19 (lowest) to -20 (highest). Only root may set the negative (higher) priorities. Related commands are renice, snice, and skill.

nohup

Keeps a command running even after user logs off. The command will run as a foreground process unless followed by &. If you use nohup within a script, consider coupling it with a wait to avoid creating an orphan or zombie process.

pidof

Identifies process id (pid) of a running job. Since job control commands, such as kill and renice act on the pid of a process (not its name), it is sometimes necessary to identify that pid. The pidof command is the approximate counterpart to the $PPID internal variable.

bash$ pidof xclock 880

Example 13-4. pidof helps kill a process

#!/bin/bash # kill-process.sh NOPROCESS=2 process=xxxyyyzzz # Use nonexistent process. # For demo purposes only… # … don’t want to actually kill any actual process with this script. # # If, for example, you wanted to use this script to logoff the Internet, # process=pppd t=`pidof $process` # Find pid (process id) of $process. # The pid is needed by ‘kill’ (can’t ‘kill’ by program name). if [ -z “$t” ] # If process not present, ‘pidof’ returns null. then echo “Process $process was not running.” echo “Nothing killed.” exit $NOPROCESS fi kill $t # May need ‘kill -9′ for stubborn process. # Need a check here to see if process allowed itself to be killed. # Perhaps another ” t=`pidof $process` “. # This entire script could be replaced by # kill $(pidof -x process_name) # but it would not be as instructive. exit 0

fuser

Identifies the processes (by pid) that are accessing a given file, set of files, or directory. May also be invoked with the -k option, which kills those processes. This has interesting implications for system security, especially in scripts preventing unauthorized users from accessing system services.

crond

Administrative program scheduler, performing such duties as cleaning up and deleting system log files and updating the slocate database. This is the superuser version of at (although each user may have their own crontab file which can be changed with the crontab command). It runs as a daemon and executes scheduled entries from /etc/crontab.

Process Control and Booting

init

The init command is the parent of all processes. Called in the final step of a bootup, init determines the runlevel of the system from /etc/inittab. Invoked by its alias telinit, and by root only.

telinit

Symlinked to init, this is a means of changing the system runlevel, usually done for system maintenance or emergency filesystem repairs. Invoked only by root. This command can be dangerous - be certain you understand it well before using!

runlevel

Shows the current and last runlevel, that is, whether the system is halted (runlevel 0), in single-user mode (1), in multi-user mode (2 or 3), in X Windows (5), or rebooting (6). This command accesses the /var/run/utmp file.

halt, shutdown, reboot

Command set to shut the system down, usually just prior to a power down.

Network

ifconfig

Network interface configuration and tuning utility. It is most often used at bootup to set up the interfaces, or to shut them down when rebooting.

# Code snippets from /etc/rc.d/init.d/network # … # Check that networking is up. [ ${NETWORKING} = “no” ] && exit 0 [ -x /sbin/ifconfig ] || exit 0 # … for i in $interfaces ; do if ifconfig $i 2>/dev/null | grep -q “UP” >/dev/null 2>&1 ; then action “Shutting down interface $i: ” ./ifdown $i boot fi # The GNU-specific “-q” option to “grep” means “quiet”, i.e., producing no output. # Redirecting output to /dev/null is therefore not strictly necessary. # … echo “Currently active devices:” echo `/sbin/ifconfig | grep ^[a-z] | awk ‘{print $1}’` # ^^^^^ should be quoted to prevent globbing. # The following also work. # echo $(/sbin/ifconfig | awk ‘/^[a-z]/ { print $1 })’ # echo $(/sbin/ifconfig | sed -e ’s/ .*//’) # Thanks, S.C., for additional comments.

route

Show info about or make changes to the kernel routing table.

bash$ route Destination Gateway Genmask Flags MSS Window irtt Iface pm3-67.bozosisp * 255.255.255.255 UH 40 0 0 ppp0 127.0.0.0 * 255.0.0.0 U 40 0 0 lo default pm3-67.bozosisp 0.0.0.0 UG 40 0 0 ppp0

chkconfig

Check network configuration. This command lists and manages the network services started at bootup in the /etc/rc?.d directory.

Originally a port from IRIX to Red Hat Linux, chkconfig may not be part of the core installation of some Linux flavors.

bash$ chkconfig –list atd 0:off 1:off 2:off 3:on 4:on 5:on 6:off rwhod 0:off 1:off 2:off 3:off 4:off 5:off 6:off …

tcpdump

Network packet “sniffer”. This is a tool for analyzing and troubleshooting traffic on a network by dumping packet headers that match specified criteria.

Dump ip packet traffic between hosts bozoville and caduceus:

bash$ tcpdump ip host bozoville and caduceus

Of course, the output of tcpdump can be parsed, using certain of the previously discussed text processing utilities.

Filesystem

mount

Mount a filesystem, usually on an external device, such as a floppy or CDROM. The file /etc/fstab provides a handy listing of available filesystems, partitions, and devices, including options, that may be automatically or manually mounted. The file /etc/mtab shows the currently mounted filesystems and partitions (including the virtual ones, such as /proc).

mount -a mounts all filesystems and partitions listed in /etc/fstab, except those with a noauto option. At bootup, a startup script in /etc/rc.d (rc.sysinit or something similar) invokes this to get everything mounted.

mount -t iso9660 /dev/cdrom /mnt/cdrom # Mounts CDROM mount /mnt/cdrom # Shortcut, if /mnt/cdrom listed in /etc/fstab

This versatile command can even mount an ordinary file on a block device, and the file will act as if it were a filesystem. Mount accomplishes that by associating the file with a loopback device. One application of this is to mount and examine an ISO9660 image before burning it onto a CDR.

Example 13-5. Checking a CD image

# As root… mkdir /mnt/cdtest # Prepare a mount point, if not already there. mount -r -t iso9660 -o loop cd-image.iso /mnt/cdtest # Mount the image. # “-o loop” option equivalent to “losetup /dev/loop0″ cd /mnt/cdtest # Now, check the image. ls -alR # List the files in the directory tree there. # And so forth.

umount

Unmount a currently mounted filesystem. Before physically removing a previously mounted floppy or CDROM disk, the device must be umounted, else filesystem corruption may result.

umount /mnt/cdrom # You may now press the eject button and safely remove the disk.

NOTE	The automount utility, if properly installed, can mount and unmount floppies or CDROM disks as they are accessed or removed. On laptops with swappable floppy and CDROM drives, this can cause problems, though.

sync

Forces an immediate write of all updated data from buffers to hard drive (synchronize drive with buffers). While not strictly necessary, a sync assures the sys admin or user that the data just changed will survive a sudden power failure. In the olden days, a sync; sync (twice, just to make absolutely sure) was a useful precautionary measure before a system reboot.

At times, you may wish to force an immediate buffer flush, as when securely deleting a file or when the lights begin to flicker.

losetup

Sets up and configures loopback devices.

Example 13-6. Creating a filesystem in a file

SIZE=1000000 # 1 meg head -c $SIZE < /dev/zero > file # Set up file of designated size. losetup /dev/loop0 file # Set it up as loopback device. mke2fs /dev/loop0 # Create filesystem. mount -o loop /dev/loop0 /mnt # Mount it. # Thanks, S.C.

mkswap

Creates a swap partition or file. The swap area must subsequently be enabled with swapon.

swapon, swapoff

Enable / disable swap partitition or file. These commands usually take effect at bootup and shutdown.

mke2fs

Create a Linux ext2 filesystem. This command must be invoked as root.

Example 13-7. Adding a new hard drive

#!/bin/bash # Adding a second hard drive to system. # Software configuration. Assumes hardware already mounted. # From an article by the author of this document. # in issue #38 of “Linux Gazette”, http://www.linuxgazette.com. ROOT_UID=0 # This script must be run as root. E_NOTROOT=67 # Non-root exit error. if [ “$UID” -ne “$ROOT_UID” ] then echo “Must be root to run this script.” exit $E_NOTROOT fi # Use with extreme caution! # If something goes wrong, you may wipe out your current filesystem. NEWDISK=/dev/hdb # Assumes /dev/hdb vacant. Check! MOUNTPOINT=/mnt/newdisk # Or choose another mount point. fdisk $NEWDISK mke2fs -cv $NEWDISK1 # Check for bad blocks & verbose output. # Note: /dev/hdb1, *not* /dev/hdb! mkdir $MOUNTPOINT chmod 777 $MOUNTPOINT # Makes new drive accessible to all users. # Now, test… # mount -t ext2 /dev/hdb1 /mnt/newdisk # Try creating a directory. # If it works, umount it, and proceed. # Final step: # Add the following line to /etc/fstab. # /dev/hdb1 /mnt/newdisk ext2 defaults 1 1 exit 0

tune2fs

Tune ext2 filesystem. May be used to change filesystem parameters, such as maximum mount count. This must be invoked as root.

WARNING

This is an extremely dangerous command. Use it at your own risk, as you may inadvertently destroy your filesystem.

dumpe2fs

Dump (list to stdout) very verbose filesystem info. This must be invoked as root.

root# dumpe2fs /dev/hda7 | grep ‘ount count’ dumpe2fs 1.19, 13-Jul-2000 for EXT2 FS 0.5b, 95/08/09 Mount count: 6 Maximum mount count: 20

hdparm

List or change hard disk parameters. This command must be invoked as root, and it may be dangerous if misused.

fdisk

Create or change a partition table on a storage device, usually a hard drive. This command must be invoked as root.

WARNING

Use this command with extreme caution. If something goes wrong, you may destroy an existing filesystem.

fsck, e2fsck, debugfs

Filesystem check, repair, and debug command set.

fsck: a front end for checking a UNIX filesystem (may invoke other utilities). The actual filesystem type generally defaults to ext2.

e2fsck: ext2 filesystem checker.

debugfs: ext2 filesystem debugger. One of the uses of this versatile, but dangerous command is to (attempt to) recover deleted files. For advanced users only!

CAUTION

All of these should be invoked as root, and they can damage or destroy a filesystem if misused.

badblocks

Checks for bad blocks (physical media flaws) on a storage device. This command finds use when formatting a newly installed hard drive or testing the integrity of backup media. As an example, badblocks /dev/fd0 tests a floppy disk.

The badblocks command may be invoked destructively (overwrite all data) or in non-destructive read-only mode. If root user owns the device to be tested, as is generally the case, then root must invoke this command.

mkbootdisk

Creates a boot floppy which can be used to bring up the system if, for example, the MBR (master boot record) becomes corrupted. The mkbootdisk command is actually a Bash script, written by Erik Troan, in the /sbin directory.

chroot

CHange ROOT directory. Normally commands are fetched from $PATH, relative to /, the default root directory. This changes the root directory to a different one (and also changes the working directory to there). This is useful for security purposes, for instance when the system administrator wishes to restrict certain users, such as those telnetting in, to a secured portion of the filesystem (this is sometimes referred to as confining a guest user to a “chroot jail”). Note that after a chroot, the execution path for system binaries is no longer valid.

A chroot /opt would cause references to /usr/bin to be translated to /opt/usr/bin. Likewise, chroot /aaa/bbb /bin/ls would redirect future instances of ls to /aaa/bbb as the base directory, rather than / as is normally the case. An alias XX ‘chroot /aaa/bbb ls’ in a user’s ~/.bashrc effectively restricts which portion of the filesystem she may run command “XX” on.

The chroot command is also handy when running from an emergency boot floppy (chroot to /dev/fd0), or as an option to lilo when recovering from a system crash. Other uses include installation from a different filesystem (an rpm option) or running a readonly filesystem from a CD ROM. Invoke only as root, and use with care.

CAUTION

It might be necessary to copy certain system files to a chrooted directory, since the normal $PATH can no longer be relied upon.

lockfile

This utility is part of the procmail package (www.procmail.org). It creates a lock file, a semaphore file that controls access to a file, device, or resource. The lock file serves as a flag that this particular file, device, or resource is in use by a particular process (“busy”), and this permits only restricted access (or no access) to other processes.

Lock files are used in such applications as protecting system mail folders from simultaneously being changed by multiple users, indicating that a modem port is being accessed, and showing that an instance of Netscape is using its cache. Scripts may check for the existence of a lock file created by a certain process to check if that process is running. Note that if a script attempts create a lock file that already exists, the script will likely hang.

Normally, applications create and check for lock files in the /var/lock directory. A script can test for the presence of a lock file by something like the following.

appname=xyzip # Application “xyzip” created lock file “/var/lock/xyzip.lock”. if [ -e “/var/lock/$appname.lock ] then …

mknod

Creates block or character device files (may be necessary when installing new hardware on the system).

tmpwatch

Automatically deletes files which have not been accessed within a specified period of time. Usually invoked by crond to remove stale log files.

MAKEDEV

Utility for creating device files. It must be run as root, and in the /dev directory.

root# ./MAKEDEV

This is a sort of advanced version of mknod.

Backup

dump, restore

The dump command is an elaborate filesystem backup utility, generally used on larger installations and networks. It reads raw disk partitions and writes a backup file in a binary format. Files to be backed up may be saved to a variety of storage media, including disks and tape drives. The restore command restores backups made with dump.

fdformat

Perform a low-level format on a floppy disk.

System Resources

ulimit

Sets an upper limit on system resources. Usually invoked with the -f option, which sets a limit on file size (ulimit -f 1000 limits files to 1 meg maximum). The -t option limits the coredump size (ulimit -c 0 eliminates coredumps). Normally, the value of ulimit would be set in /etc/profile and/or ~/.bash_profile.

umask

User file creation MASK. Limit the default file attributes for a particular user. All files created by that user take on the attributes specified by umask. The (octal) value passed to umask defines the file permissions disabled. For example, umask 022 ensures that new files will have at most 755 permissions (777 NAND 022). [6] Of course, the user may later change the attributes of particular files with chmod. The usual practice is to set the value of umask in /etc/profile and/or ~/.bash_profile.

rdev

Get info about or make changes to root device, swap space, or video mode. The functionality of rdev has generally been taken over by lilo, but rdev remains useful for setting up a ram disk. This is another dangerous command, if misused.

Modules

lsmod

List installed kernel modules.

bash$ lsmod Module Size Used by autofs 9456 2 (autoclean) opl3 11376 0 serial_cs 5456 0 (unused) sb 34752 0 uart401 6384 0 [sb] sound 58368 0 [opl3 sb uart401] soundlow 464 0 [sound] soundcore 2800 6 [sb sound] ds 6448 2 [serial_cs] i82365 22928 2 pcmcia_core 45984 0 [serial_cs ds i82365]

NOTE	Doing a cat /proc/modules gives the same information.

insmod

Force installation of a kernel module (use modprobe instead, when possible). Must be invoked as root.

rmmod

Force unloading of a kernel module. Must be invoked as root.

modprobe

Module loader that is normally invoked automatically in a startup script. Must be invoked as root.

depmod

Creates module dependency file, usually invoked from startup script.

Miscellaneous

env

Runs a program or script with certain environmental variables set or changed (without changing the overall system environment). The [varname=xxx] permits changing the environmental variable varname for the duration of the script. With no options specified, this command lists all the environmental variable settings.

NOTE		In Bash and other Bourne shell derivatives, it is possible to set variables in a single command’s environment. var1=value1 var2=value2 commandXXX # $var1 and $var2 set in the environment of ‘commandXXX’ only.
TIP	The first line of a script (the “sha-bang” line) may use env when the path to the shell or interpreter is unknown. #! /usr/bin/env perl print “This Perl script will run,n”; print “even when I don’t know where to find Perl.n”; # Good for portable cross-platform scripts, # where the Perl binaries may not be in the expected place. # Thanks, S.C.

ldd

Show shared lib dependencies for an executable file.

bash$ ldd /bin/ls libc.so.6 => /lib/libc.so.6 (0×4000c000) /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0×80000000)

strip

Remove the debugging symbolic references from an executable binary. This decreases its size, but makes debugging it impossible.

This command often occurs in a Makefile, but rarely in a shell script.

nm

List symbols in an unstripped compiled binary.

rdist

Remote distribution client: synchronizes, clones, or backs up a file system on a remote server.

Using our knowledge of administrative commands, let us examine a system script. One of the shortest and simplest to understand scripts is killall, used to suspend running processes at system shutdown.

Example 13-8. killall, from /etc/rc.d/init.d

#!/bin/sh # –> Comments added by the author of this document marked by “# –>”. # –> This is part of the ‘rc’ script package # –> by Miquel van Smoorenburg, # –> This particular script seems to be Red Hat specific # –> (may not be present in other distributions). # Bring down all unneeded services that are still running (there shouldn’t # be any, so this is just a sanity check) for i in /var/lock/subsys/*; do # –> Standard for/in loop, but since “do” is on same line, # –> it is necessary to add “;”. # Check if the script is there. [ ! -f $i ] && continue # –> This is a clever use of an “and list”, equivalent to: # –> if [ ! -f “$i” ]; then continue # Get the subsystem name. subsys=${i#/var/lock/subsys/} # –> Match variable name, which, in this case, is the file name. # –> This is the exact equivalent of subsys=`basename $i`. # –> It gets it from the lock file name (if there is a lock file, # –>+ that’s proof the process has been running). # –> See the “lockfile” entry, above. # Bring the subsystem down. if [ -f /etc/rc.d/init.d/$subsys.init ]; then /etc/rc.d/init.d/$subsys.init stop else /etc/rc.d/init.d/$subsys stop # –> Suspend running jobs and daemons # –> using the ’stop’ shell builtin. fi done

More at http://www.die.net/doc/linux/man/