Chapter 13. System and Administrative Commands

Users and 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.

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 [2] and deletes associated files.

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

Also see Example 9-4.

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. See Example A-10.

users

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

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.

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

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

Also see Example 9-3.

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 = <undef>; eol2 = <undef>; 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 wha<ctl-W>I<ctl-H>foo bar<ctl-U>hello world<ENTER> <ctl-D> bash$ cat filexxx hello world bash$ bash$ wc -c < file 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. Stephane Chazelas

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.

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!

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.

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 10-24.

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.

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) = 0x804f5e4 ...

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

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

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.

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 cron 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 (see Example 11-8 and Example 28-1).

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 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.

See also Example 30-5.

route

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

bash$ route Destination Gateway Genmask Flags Metric Ref Use Iface 127.0.0.0 * 255.0.0.0 U 0 0 0 lo

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. [3]

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.

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 (see Example 12-31) 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", https://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

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

tune2fs

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.

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.

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.

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. [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.

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.

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).

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. [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.

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 (see Chapter 27).

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 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).

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]

insmod

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

modprobe

Module loader that is normally invoked automatically in a startup script.

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.

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.

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 (0x4000c000) /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x80000000)

strip

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