Chapter 13. System and Administrative Commands

The startup and shutdown scripts in /etc/rc.d illustrate the uses (and usefulness) of many of these comands. 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


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


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
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. [1]

 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.
   1 chgrp --recursive dunderheads *.data
   2 #  The "dunderheads" group will now own all the "*.data" files
   3 #+ 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 [2] and deletes associated files.


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


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

Also see Example 9-5.


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


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

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

 bash$ logname
 bash$ whoami


 bash$ su
 Password: ......
 bash# whoami
 bash# logname

Runs a program or script as a substitute user. su rjones starts a shell as user rjones. A naked su defaults to root. See Example A-16.


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

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

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


Sets or changes a user's password.

The passwd can be used in a script, but should not be.
   1 #!/bin/bash
   2 # Not a good idea.
   3 #  This script must be run as root,
   4 #+ or better yet, not run at all.
   6 ROOT_UID=0         # Root has $UID 0.
   7 E_WRONG_USER=65    # Not root?
   9 if [ "$UID" -ne "$ROOT_UID" ]
  10 then
  11   echo; echo "Only root can run this script."; echo
  12   exit $E_WRONG_USER
  13 else
  14   echo; echo "You should know better than to run this script, root."
  15 fi  
  18 username=bozo
  19 NEWPASSWORD=security_violation
  21 echo "$NEWPASSWORD" | passwd --stdin "$username"
  22 #  The '--stdin' option to 'passwd' permits
  23 #+ getting new password from stdin (or a pipe).
  25 echo; echo "User $username's password changed!"
  27 # Using the 'passwd' command in a script is dangerous.
  29 exit 0


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

 bash$ ac
         total       68.08

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


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.



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

 bash$ tty

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

   1 #!/bin/bash
   2 # Using "stty" to set an erase character when reading input.
   4 echo -n "What is your name? "
   5 read name                      # Try to erase characters of input.
   6                                # Won't work.
   7 echo "Your name is $name."
   9 stty erase '#'                 # Set "hashmark" (#) as erase character.
  10 echo -n "What is your name? "
  11 read name                      # Use # to erase last character typed.
  12 echo "Your name is $name."
  14 exit 0

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

   1 #!/bin/bash
   3 echo
   4 echo -n "Enter password "
   5 read passwd
   6 echo "password is $passwd"
   7 echo -n "If someone had been looking over your shoulder, "
   8 echo "your password would have been compromised."
  10 echo && echo  # Two line-feeds in an "and list".
  12 stty -echo    # Turns off screen echo.
  14 echo -n "Enter password again "
  15 read passwd
  16 echo
  17 echo "password is $passwd"
  18 echo
  20 stty echo     # Restores screen echo.
  22 exit 0

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

Example 13-3. Keypress detection

   1 #!/bin/bash
   2 # Detect a user keypress ("hot keyboard").
   4 echo
   6 old_tty_settings=$(stty -g)   # Save old settings.
   7 stty -icanon
   8 Keypress=$(head -c1)          # or $(dd bs=1 count=1 2> /dev/null)
   9                               # on non-GNU systems
  11 echo
  12 echo "Key pressed was \""$Keypress"\"."
  13 echo
  15 stty "$old_tty_settings"      # Restore old settings.
  17 # Thanks, Stephane Chazelas.
  19 exit 0

Also see Example 9-3.


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).


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

   1 # From /etc/pcmcia/serial script:
   3 IRQ=`setserial /dev/$DEVICE | sed -e 's/.*IRQ: //'`
   4 setserial /dev/$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.


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.


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.


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!


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


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 < hda5 hda6 hda7 > hda4

Information and Statistics


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

Show system architecture. Equivalent to uname -m. See Example 10-26.

 bash$ arch
 bash$ uname -m

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.


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


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


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
 init         1    root  mem    REG        3,5   30748    30303 /sbin/init
 init         1    root  mem    REG        3,5   73120     8069 /lib/
 init         1    root  mem    REG        3,5  931668     8075 /lib/
 cardmgr    213    root  mem    REG        3,5   36956    30357 /sbin/cardmgr


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)                                  = 0x804f5e4

This is the Linux equivalent of truss.


Network port scanner. This command scans a server to locate open ports and the services associated with those ports. It is an important security tool for locking down a network against hacking attempts.

   1 #!/bin/bash
   3 SERVER=$HOST                           # localhost.localdomain (
   4 PORT_NUMBER=25                         # SMTP port.
   6 nmap $SERVER | grep -w "$PORT_NUMBER"  # Is that particular port open?
   7 #              grep -w matches whole words only,
   8 #+             so this wouldn't match port 1025, for example.
  10 exit 0
  12 # 25/tcp     open        smtp


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 }'

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

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


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

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

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

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


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


Show current network statistics and information, such as routing tables and active connections. This utility accesses information in /proc/net (Chapter 28). See Example 28-2.

netstat -r is equivalent to route.


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

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
 bash$ echo $HOSTNAME

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

 bash$ hostid


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               localhost.localdomain localhost

As it happens, transposing the bytes of, we get, 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.


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

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)
   . . .

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


Appends a user-generated message to the system log (/var/log/messages). You do not have to be root to invoke logger.
   1 logger Experiencing instability in network connection at 23:10, 05/21.
   2 # 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.
   1 logger -t $0 -i Logging at line "$LINENO".
   2 # The "-t" option specifies the tag for the logger entry.
   3 # The "-i" option records the process ID.
   5 # tail /var/log/message
   6 # ...
   7 # Jul  7 20:48:58 localhost ./[1712]: Logging at line 3.


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


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 (see Example 11-10 and Example 28-1).

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

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


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

   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


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.


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.


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

Example 13-4. pidof helps kill a process

   1 #!/bin/bash
   2 #
   6 process=xxxyyyzzz  # Use nonexistent process.
   7 # For demo purposes only...
   8 # ... don't want to actually kill any actual process with this script.
   9 #
  10 # If, for example, you wanted to use this script to logoff the Internet,
  11 #     process=pppd
  13 t=`pidof $process`       # Find pid (process id) of $process.
  14 # The pid is needed by 'kill' (can't 'kill' by program name).
  16 if [ -z "$t" ]           # If process not present, 'pidof' returns null.
  17 then
  18   echo "Process $process was not running."
  19   echo "Nothing killed."
  20   exit $NOPROCESS
  21 fi  
  23 kill $t                  # May need 'kill -9' for stubborn process.
  25 # Need a check here to see if process allowed itself to be killed.
  26 # Perhaps another " t=`pidof $process` ".
  29 # This entire script could be replaced by
  30 #    kill $(pidof -x process_name)
  31 # but it would not be as instructive.
  33 exit 0


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.


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


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.


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!


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 interface configuration and tuning utility. It is most often used at bootup to set up the interfaces, or to shut them down when rebooting.
   1 # Code snippets from /etc/rc.d/init.d/network
   3 # ...
   5 # Check that networking is up.
   6 [ ${NETWORKING} = "no" ] && exit 0
   8 [ -x /sbin/ifconfig ] || exit 0
  10 # ...
  12 for i in $interfaces ; do
  13   if ifconfig $i 2>/dev/null | grep -q "UP" >/dev/null 2>&1 ; then
  14     action "Shutting down interface $i: " ./ifdown $i boot
  15   fi
  16 # The GNU-specific "-q" option to "grep" means "quiet", i.e., producing no output.
  17 # Redirecting output to /dev/null is therefore not strictly necessary.
  19 # ...
  21 echo "Currently active devices:"
  22 echo `/sbin/ifconfig | grep ^[a-z] | awk '{print $1}'`
  23 #                            ^^^^^  should be quoted to prevent globbing.
  24 #  The following also work.
  25 #    echo $(/sbin/ifconfig | awk '/^[a-z]/ { print $1 })'
  26 #    echo $(/sbin/ifconfig | sed -e 's/ .*//')
  27 #  Thanks, S.C., for additional comments.
See also Example 30-6.


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

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


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


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.



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.

   1 mount -t iso9660 /dev/cdrom /mnt/cdrom
   2 # Mounts CDROM
   3 mount /mnt/cdrom
   4 # 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. [3]

Example 13-5. Checking a CD image

   1 # As root...
   3 mkdir /mnt/cdtest  # Prepare a mount point, if not already there.
   5 mount -r -t iso9660 -o loop cd-image.iso /mnt/cdtest   # Mount the image.
   6 #                  "-o loop" option equivalent to "losetup /dev/loop0"
   7 cd /mnt/cdtest     # Now, check the image.
   8 ls -alR            # List the files in the directory tree there.
   9                    # And so forth.


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


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.


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 (see Example 12-42) or when the lights begin to flicker.


Sets up and configures loopback devices.

Example 13-6. Creating a filesystem in a file

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


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.


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

Example 13-7. Adding a new hard drive

   1 #!/bin/bash
   3 # Adding a second hard drive to system.
   4 # Software configuration. Assumes hardware already mounted.
   5 # From an article by the author of this document.
   6 # in issue #38 of "Linux Gazette",
   8 ROOT_UID=0     # This script must be run as root.
   9 E_NOTROOT=67   # Non-root exit error.
  11 if [ "$UID" -ne "$ROOT_UID" ]
  12 then
  13   echo "Must be root to run this script."
  14   exit $E_NOTROOT
  15 fi  
  17 # Use with extreme caution!
  18 # If something goes wrong, you may wipe out your current filesystem.
  21 NEWDISK=/dev/hdb         # Assumes /dev/hdb vacant. Check!
  22 MOUNTPOINT=/mnt/newdisk  # Or choose another mount point.
  25 fdisk $NEWDISK
  26 mke2fs -cv $NEWDISK1   # Check for bad blocks & verbose output.
  27 #  Note:    /dev/hdb1, *not* /dev/hdb!
  28 mkdir $MOUNTPOINT
  29 chmod 777 $MOUNTPOINT  # Makes new drive accessible to all users.
  32 # Now, test...
  33 # mount -t ext2 /dev/hdb1 /mnt/newdisk
  34 # Try creating a directory.
  35 # If it works, umount it, and proceed.
  37 # Final step:
  38 # Add the following line to /etc/fstab.
  39 # /dev/hdb1  /mnt/newdisk  ext2  defaults  1 1
  41 exit 0

See also Example 13-6 and Example 29-3.


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


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


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

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


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


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!


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


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. [4] 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.


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.


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.


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


This utility is part of the procmail package ( 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.
   1 appname=xyzip
   2 # Application "xyzip" created lock file "/var/lock/xyzip.lock".
   4 if [ -e "/var/lock/$appname.lock ]
   5 then
   6   ...


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


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


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.


dump, restore

The dump command is an elaborate filesystem backup utility, generally used on larger installations and networks. [5] 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.


Perform a low-level format on a floppy disk.

System Resources


Sets an upper limit on use of 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 (see Chapter 27).


Judicious use of ulimit can protect a system against the dreaded fork bomb.

   1 #!/bin/bash
   3 while 1     #  Endless loop.
   4 do
   5   $0 &      #  This script invokes itself . . .
   6             #+ forks an infinite number of times . . .
   7             #+ until the system freezes up because all resources exhausted.
   8 done        #  This is the notorious "sorcerer's appentice" scenario.	   
  10 exit 0      #  Will not exit here, because this script will never terminate.

A ulimit -Hu XX (where XX is the user process limit) in /etc/profile would abort this script when it exceeds the preset limit.


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 (see Chapter 27).


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.



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]


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


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


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


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


Creates module dependency file, usually invoked from startup script.



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.


In Bash and other Bourne shell derivatives, it is possible to set variables in a single command's environment.
   1 var1=value1 var2=value2 commandXXX
   2 # $var1 and $var2 set in the environment of 'commandXXX' only.


The first line of a script (the "sha-bang" line) may use env when the path to the shell or interpreter is unknown.
   1 #! /usr/bin/env perl
   3 print "This Perl script will run,\n";
   4 print "even when I don't know where to find Perl.\n";
   6 # Good for portable cross-platform scripts,
   7 # where the Perl binaries may not be in the expected place.
   8 # Thanks, S.C.


Show shared lib dependencies for an executable file.

 bash$ ldd /bin/ls => /lib/ (0x4000c000)
/lib/ => /lib/ (0x80000000)

Run a command repeatedly, at specified time intervals.

The default is two-second intervals, but this may be changed with the -n option.

   1 watch -n 5 tail /var/log/messages
   2 # Shows tail end of system log, /var/log/messages, every five seconds.


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.


List symbols in an unstripped compiled binary.


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

   1 #!/bin/sh
   3 # --> Comments added by the author of this document marked by "# -->".
   5 # --> This is part of the 'rc' script package
   6 # --> by Miquel van Smoorenburg, <>
   8 # --> This particular script seems to be Red Hat specific
   9 # --> (may not be present in other distributions).
  11 # Bring down all unneeded services that are still running (there shouldn't 
  12 # be any, so this is just a sanity check)
  14 for i in /var/lock/subsys/*; do
  15         # --> Standard for/in loop, but since "do" is on same line,
  16         # --> it is necessary to add ";".
  17 	# Check if the script is there.
  18 	[ ! -f $i ] && continue
  19 	# --> This is a clever use of an "and list", equivalent to:
  20 	# --> if [ ! -f "$i" ]; then continue
  22 	# Get the subsystem name.
  23 	subsys=${i#/var/lock/subsys/}
  24 	# --> Match variable name, which, in this case, is the file name.
  25 	# --> This is the exact equivalent of subsys=`basename $i`.
  27 	# -->  It gets it from the lock file name (if there is a lock file,
  28 	# -->+ that's proof the process has been running).
  29 	# -->  See the "lockfile" entry, above.
  32 	# Bring the subsystem down.
  33 	if [ -f /etc/rc.d/init.d/$subsys.init ]; then
  34 	    /etc/rc.d/init.d/$subsys.init stop
  35 	else
  36 	    /etc/rc.d/init.d/$subsys stop
  37         # --> Suspend running jobs and daemons
  38         # --> using the 'stop' shell builtin.
  39 	fi
  40 done

That wasn't so bad. Aside from a little fancy footwork with variable matching, there is no new material there.

Exercise 1. In /etc/rc.d/init.d, analyze the halt script. It is a bit longer than killall, but similar in concept. Make a copy of this script somewhere in your home directory and experiment with it (do not run it as root). Do a simulated run with the -vn flags (sh -vn scriptname). Add extensive comments. Change the "action" commands to "echos".

Exercise 2. Look at some of the more complex scripts in /etc/rc.d/init.d. See if you can understand parts of them. Follow the above procedure to analyze them. For some additional insight, you might also examine the file sysvinitfiles in /usr/share/doc/initscripts-?.??, which is part of the "initscripts" documentation.



This is the case on a Linux machine or a UNIX system with disk quotas.


The userdel command will fail if the particular user being deleted is still logged on.


For more detail on burning CDRs, see Alex Withers' article, Creating CDs, in the October, 1999 issue of Linux Journal.


The -c option to mke2fs also invokes a check for bad blocks.


Operators of single-user Linux systems generally prefer something simpler for backups, such as tar.


NAND is the logical "not-and" operator. Its effect is somewhat similar to subtraction.