Table of Contents
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.
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 $GROUPS internal variable, but gives the group names, rather than the numbers.
bash$groupsbozita cdrom cdwriter audio xgrpbash$echo $GROUPS501
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. [88]
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).
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 [89] and deletes associated files.
The adduser command is a synonym for useradd and is usually a symbolic link to it.
Modify a user account. Changes may be made to the password, group membership, expiration date, and other attributes of a given user's account. With this command, a user's password may be locked, which has the effect of disabling the account.
Modify a given group. The group name and/or ID number may be changed using this command.
The id command lists the real and effective user IDs and the group IDs of the user associated with the current process. This is the counterpart to the $UID, $EUID, and $GROUPS internal Bash variables.
bash$iduid=501(bozo) gid=501(bozo) groups=501(bozo),22(cdrom),80(cdwriter),81(audio)bash$echo $UID501
The id command shows the effective IDs only when they differ from the real ones.
Also see Example 9.5, “Am I root?”.
The lid (list ID) command shows the group(s) that a given user belongs to, or alternately, the users belonging to a given group. May be invoked only by root.
root#lid bozobozo(gid=500)root#lid daemonbin(gid=1) daemon(gid=2) adm(gid=4) lp(gid=7)
Show all users logged on to the system.
bash$whobozo 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$who -mlocalhost.localdomain!bozo pts/2 Apr 27 17:49
whoami is similar to who -m, but only lists the user name.
bash$whoamibozo
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$w | grep startxbozo tty1 - 4:22pm 6:41 4.47s 0.45s startx
Show current user's login name (as found in
/var/run/utmp). This is a
near-equivalent to whoami,
above.
bash$lognamebozobash$whoamibozo
However . . .
bash$suPassword: ......bash#whoamirootbash#lognamebozo
While logname prints the name of the logged in user, whoami gives the name of the user attached to the current process. As we have just seen, sometimes these are not the same.
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.14, “fifo: Making daily backups, using named pipes”.
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.
Sets, changes, or manages a user's password.
The passwd command can be used in a script, but probably should not be.
Example 17.1. Setting a new password
#!/bin/bash # setnew-password.sh: For demonstration purposes only. # Not a good idea to actually run this script. # This script must be run as root. ROOT_UID=0 # Root has $UID 0. E_WRONG_USER=65 # Not root? E_NOSUCHUSER=70 SUCCESS=0 if [ "$UID" -ne "$ROOT_UID" ] then echo; echo "Only root can run this script."; echo exit $E_WRONG_USER else echo echo "You should know better than to run this script, root." echo "Even root users get the blues... " echo fi username=bozo NEWPASSWORD=security_violation # Check if bozo lives here. grep -q "$username" /etc/passwd if [ $? -ne $SUCCESS ] then echo "User $username does not exist." echo "No password changed." exit $E_NOSUCHUSER fi echo "$NEWPASSWORD" | passwd --stdin "$username" # The '--stdin' option to 'passwd' permits #+ getting a new password from stdin (or a pipe). echo; echo "User $username's password changed!" # Using the 'passwd' command in a script is dangerous. exit 0
The passwd command's -l,
-u, and -d options permit
locking, unlocking, and deleting a user's password. Only
root may use these options.
Show users' logged in time, as read from
/var/log/wtmp. This is one of the GNU
accounting utilities.
bash$actotal 68.08
List last logged in users, as read from
/var/log/wtmp. This command can also
show remote logins.
For example, to show the last few times the system rebooted:
bash$last rebootreboot system boot 2.6.9-1.667 Fri Feb 4 18:18 (00:02) reboot system boot 2.6.9-1.667 Fri Feb 4 15:20 (01:27) reboot system boot 2.6.9-1.667 Fri Feb 4 12:56 (00:49) reboot system boot 2.6.9-1.667 Thu Feb 3 21:08 (02:17) . . . wtmp begins Tue Feb 1 12:50:09 2005
Change user's group ID without logging out. This permits access to the new group's files. Since users may be members of multiple groups simultaneously, this command finds only limited use.
Kurt Glaesemann points out that the newgrp command could prove helpful in setting the default group permissions for files a user writes. However, the chgrp command might be more convenient for this purpose.
Echoes the name (filename) of the current user's terminal. Note that each separate xterm window counts as a different terminal.
bash$tty/dev/pts/1
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 17.2. 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 backspace
#+ to erase characters of input.
# Problems?
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
# Even after the script exits, the new key value remains set.
# Exercise: How would you reset the erase character to the default value?
Example 17.3. secret password: Turning off terminal echoing
#!/bin/bash # secret-pw.sh: secret password 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. # May also be done with # read -sp passwd # A big Thank You to Leigh James for pointing this out. echo -n "Enter password again " read passwd echo echo "password is $passwd" echo stty echo # Restores screen echo. exit 0 # Do an 'info stty' for more on this useful-but-tricky command.
A creative use of stty is detecting a user keypress (without hitting ENTER).
Example 17.4. Keypress detection
#!/bin/bash
# keypress.sh: Detect a user keypress ("hot keys").
echo
old_tty_settings=$(stty -g) # Save old settings (why?).
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 "$old_tty_settings" # Restore old settings.
# Thanks, Stephane Chazelas.
exit 0
Also see Example 9.3, “Once more, timed input” and Example A.43, “A command-line stopwatch”.
Set certain terminal attributes. This command writes
to its terminal's stdout a string that
changes the behavior of that terminal.
bash$setterm -cursor offbash$
The setterm command can be used within a
script to change the appearance of text written to
stdout, although there are certainly
better tools available
for this purpose.
setterm -bold on echo bold hello setterm -bold off echo normal hello
Show or initialize terminal settings. This is a less capable version of stty.
bash$tset -rTerminal type is xterm-xfree86. Kill is control-U (^U). Interrupt is control-C (^C).
Set or display serial port parameters. This command must be run by root and is usually found in a system setup script.
# From /etc/pcmcia/serial script: IRQ=`setserial /dev/$DEVICE | sed -e 's/.*IRQ: //'` setserial /dev/$DEVICE irq 0 ; setserial /dev/$DEVICE irq $IRQ
The initialization process for a terminal uses getty or agetty to set it up for login by a user. These commands are not used within user shell scripts. Their scripting counterpart is stty.
Enables or disables write access to the current user's terminal. Disabling access would prevent another user on the network to write to the terminal.
It can be quite annoying to have a message about ordering pizza suddenly appear in the middle of the text file you are editing. On a multi-user network, you might therefore wish to disable write access to your terminal when you need to avoid interruptions.
This is an acronym for “write all,” i.e., sending a message to all users at every terminal logged into the network. It is primarily a system administrator's tool, useful, for example, when warning everyone that the system will shortly go down due to a problem (see Example 19.1, “broadcast: Sends message to everyone logged in”).
bash$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 that terminal.
Output system specifications (OS, kernel version,
etc.) to stdout. Invoked with the
-a option, gives verbose system info
(see Example 16.5, “Logfile: Using xargs to monitor system log”). The -s
option shows only the OS type.
bash$unameLinuxbash$uname -sLinuxbash$uname -aLinux iron.bozo 2.6.15-1.2054_FC5 #1 Tue Mar 14 15:48:33 EST 2006 i686 i686 i386 GNU/Linux
Show system architecture. Equivalent to uname -m. See Example 11.27, “Using command substitution to generate the case variable”.
bash$archi686bash$uname -mi686
Gives information about previous commands, as stored
in the /var/account/pacct file. Command
name and user name can be specified by options. This is
one of the GNU accounting utilities.
List the last login time of all system users. This
references the /var/log/lastlog
file.
bash$lastlogroot 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 2001bash$lastlog | grep rootroot tty1 Fri Dec 7 18:43:21 -0700 2001
This command will fail if the user invoking
it does not have read permission for the
/var/log/lastlog file.
List open files. This command outputs a detailed table of all currently open files and gives information about their owner, size, the processes associated with them, and more. Of course, lsof may be piped to grep and/or awk to parse and analyze its results.
bash$lsofCOMMAND 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 ...
The lsof command is a useful,
if complex administrative tool. If you are unable to
dismount a filesystem and get an error message that it is
still in use, then running lsof helps
determine which files are still open on that filesystem. The
-i option lists open network socket files,
and this can help trace intrusion or hack attempts.
bash$lsof -an -i tcpCOMMAND PID USER FD TYPE DEVICE SIZE NODE NAME firefox 2330 bozo 32u IPv4 9956 TCP 66.0.118.137:57596->67.112.7.104:http ... firefox 2330 bozo 38u IPv4 10535 TCP 66.0.118.137:57708->216.79.48.24:http ...
See Example 30.2, “IP addresses” for an effective use of lsof.
System trace: diagnostic and debugging tool for tracing system calls and signals. This command and ltrace, following, are useful for diagnosing why a given program or package fails to run . . . perhaps due to missing libraries or related causes.
bash$strace dfexecve("/bin/df", ["df"], [/* 45 vars */]) = 0 uname({sys="Linux", node="bozo.localdomain", ...}) = 0 brk(0) = 0x804f5e4 ...
This is the Linux equivalent of the Solaris truss command.
Library trace: diagnostic and debugging tool that traces library calls invoked by a given command.
bash$ltrace df__libc_start_main(0x804a910, 1, 0xbfb589a4, 0x804fb70, 0x804fb68 <unfinished ...> setlocale(6, "") = "en_US.UTF-8" bindtextdomain("coreutils", "/usr/share/locale") = "/usr/share/locale" textdomain("coreutils") = "coreutils" __cxa_atexit(0x804b650, 0, 0, 0x8052bf0, 0xbfb58908) = 0 getenv("DF_BLOCK_SIZE") = NULL ...
The nc (netcat) utility is a complete toolkit for connecting to and listening to TCP and UDP ports. It is useful as a diagnostic and testing tool and as a component in simple script-based HTTP clients and servers.
bash$nc localhost.localdomain 25220 localhost.localdomain ESMTP Sendmail 8.13.1/8.13.1; Thu, 31 Mar 2005 15:41:35 -0700
A real-life usage example.
Example 17.5. Checking a remote server for identd
#! /bin/sh
## Duplicate DaveG's ident-scan thingie using netcat. Oooh, he'll be p*ssed.
## Args: target port [port port port ...]
## Hose stdout _and_ stderr together.
##
## Advantages: runs slower than ident-scan, giving remote inetd less cause
##+ for alarm, and only hits the few known daemon ports you specify.
## Disadvantages: requires numeric-only port args, the output sleazitude,
##+ and won't work for r-services when coming from high source ports.
# Script author: Hobbit <hobbit@avian.org>
# Used in ABS Guide with permission.
# ---------------------------------------------------
E_BADARGS=65 # Need at least two args.
TWO_WINKS=2 # How long to sleep.
THREE_WINKS=3
IDPORT=113 # Authentication "tap ident" port.
RAND1=999
RAND2=31337
TIMEOUT0=9
TIMEOUT1=8
TIMEOUT2=4
# ---------------------------------------------------
case "${2}" in
"" ) echo "Need HOST and at least one PORT." ; exit $E_BADARGS ;;
esac
# Ping 'em once and see if they *are* running identd.
nc -z -w $TIMEOUT0 "$1" $IDPORT || \
{ echo "Oops, $1 isn't running identd." ; exit 0 ; }
# -z scans for listening daemons.
# -w $TIMEOUT = How long to try to connect.
# Generate a randomish base port.
RP=`expr $$ % $RAND1 + $RAND2`
TRG="$1"
shift
while test "$1" ; do
nc -v -w $TIMEOUT1 -p ${RP} "$TRG" ${1} < /dev/null > /dev/null &
PROC=$!
sleep $THREE_WINKS
echo "${1},${RP}" | nc -w $TIMEOUT2 -r "$TRG" $IDPORT 2>&1
sleep $TWO_WINKS
# Does this look like a lamer script or what . . . ?
# ABS Guide author comments: "Ain't really all that bad . . .
#+ kinda clever, actually."
kill -HUP $PROC
RP=`expr ${RP} + 1`
shift
done
exit $?
# Notes:
# -----
# Try commenting out line 30 and running this script
#+ with "localhost.localdomain 25" as arguments.
# For more of Hobbit's 'nc' example scripts,
#+ look in the documentation:
#+ the /usr/share/doc/nc-X.XX/scripts directory.
And, of course, there's Dr. Andrew Tridgell's notorious one-line script in the BitKeeper Affair:
echo clone | nc thunk.org 5000 > e2fsprogs.dat
Shows memory and cache usage in tabular form. The output of this command lends itself to parsing, using grep, awk or Perl. The procinfo command shows all the information that free does, and much more.
bash$freetotal 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 $4 }'1880
Extract and list information and statistics from the
/proc
pseudo-filesystem. This gives a very extensive and
detailed listing.
bash$procinfo | grep BootupBootup: 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$lsdevDevice DMA IRQ I/O Ports ------------------------------------------------ cascade 4 2 dma 0080-008f dma1 0000-001f dma2 00c0-00df fpu 00f0-00ff ide0 14 01f0-01f7 03f6-03f6 ...
Show (disk) file usage, recursively. Defaults to current working directory, unless otherwise specified.
bash$du -ach1.0k ./wi.sh 1.0k ./tst.sh 1.0k ./random.file 6.0k . 6.0k total
Shows filesystem usage in tabular form.
bash$dfFilesystem 1k-blocks Used Available Use% Mounted on /dev/hda5 273262 92607 166547 36% / /dev/hda8 222525 123951 87085 59% /home /dev/hda7 1408796 1075744 261488 80% /usr
Lists all system bootup messages to
stdout. Handy for debugging and
ascertaining which device drivers were installed
and which system interrupts in use. The output
of dmesg may, of course, be
parsed with grep,
sed, or awk from within a script.
bash$dmesg | grep hdaKernel 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
Gives detailed and verbose statistics on a given file (even a directory or device file) or set of files.
bash$stat test.cruFile: "test.cru" Size: 49970 Allocated Blocks: 100 Filetype: Regular File Mode: (0664/-rw-rw-r--) Uid: ( 501/ bozo) Gid: ( 501/ bozo) Device: 3,8 Inode: 18185 Links: 1 Access: Sat Jun 2 16:40:24 2001 Modify: Sat Jun 2 16:40:24 2001 Change: Sat Jun 2 16:40:24 2001
If the target file does not exist, stat returns an error message.
bash$stat nonexistent-filenonexistent-file: No such file or directory
In a script, you can use stat to extract information about files (and filesystems) and set variables accordingly.
#!/bin/bash # fileinfo2.sh # Per suggestion of Joël Bourquard and . . . # http://www.linuxquestions.org/questions/showthread.php?t=410766 FILENAME=testfile.txt file_name=$(stat -c%n "$FILENAME") # Same as "$FILENAME" of course. file_owner=$(stat -c%U "$FILENAME") file_size=$(stat -c%s "$FILENAME") # Certainly easier than using "ls -l $FILENAME" #+ and then parsing with sed. file_inode=$(stat -c%i "$FILENAME") file_type=$(stat -c%F "$FILENAME") file_access_rights=$(stat -c%A "$FILENAME") echo "File name: $file_name" echo "File owner: $file_owner" echo "File size: $file_size" echo "File inode: $file_inode" echo "File type: $file_type" echo "File access rights: $file_access_rights" exit 0 sh fileinfo2.sh File name: testfile.txt File owner: bozo File size: 418 File inode: 1730378 File type: regular file File access rights: -rw-rw-r--
Display virtual memory statistics.
bash$vmstatprocs 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
Shows how long the system has been running, along with associated statistics.
bash$uptime10:28pm up 1:57, 3 users, load average: 0.17, 0.34, 0.27
A load average of 1 or less indicates that the system handles processes immediately. A load average greater than 1 means that processes are being queued. When the load average gets above 3 (on a single-core processor), then system performance is significantly degraded.
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 $HOSTNAME internal
variable.
bash$hostnamelocalhost.localdomainbash$echo $HOSTNAMElocalhost.localdomain
Similar to the hostname command are the domainname, dnsdomainname, nisdomainname, and ypdomainname commands. Use these to display or set the system DNS or NIS/YP domain name. Various options to hostname also perform these functions.
Echo a 32-bit hexadecimal numerical identifier for the host machine.
bash$hostid7f0100
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$cat /etc/hosts127.0.0.1 localhost.localdomain localhost
As it happens, transposing the bytes of
127.0.0.1, we get
0.127.1.0, which translates in
hex to 007f0100, the exact equivalent
of what hostid returns, above. There
exist only a few million other Linux machines with this
identical hostid.
Invoking sar (System Activity Reporter) gives a very detailed rundown on system statistics. The Santa Cruz Operation (“Old” SCO) released sar as Open Source in June, 1999.
This command is not part of the base Linux distribution, but may be obtained as part of the sysstat utilities package, written by Sebastien Godard.
bash$sarLinux 2.4.9 (brooks.seringas.fr) 09/26/03 10:30:00 CPU %user %nice %system %iowait %idle 10:40:00 all 2.21 10.90 65.48 0.00 21.41 10:50:00 all 3.36 0.00 72.36 0.00 24.28 11:00:00 all 1.12 0.00 80.77 0.00 18.11 Average: all 2.23 3.63 72.87 0.00 21.27 14:32:30 LINUX RESTART 15:00:00 CPU %user %nice %system %iowait %idle 15:10:00 all 8.59 2.40 17.47 0.00 71.54 15:20:00 all 4.07 1.00 11.95 0.00 82.98 15:30:00 all 0.79 2.94 7.56 0.00 88.71 Average: all 6.33 1.70 14.71 0.00 77.26
Show information and statistics about a designated elf binary. This is part of the binutils package.
bash$readelf -h /bin/bashELF Header: Magic: 7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00 Class: ELF32 Data: 2's complement, little endian Version: 1 (current) OS/ABI: UNIX - System V ABI Version: 0 Type: EXEC (Executable file) . . .
The size [/path/to/binary] command gives the segment sizes of a binary executable or archive file. This is mainly of use to programmers.
bash$size /bin/bashtext data bss dec hex filename 495971 22496 17392 535859 82d33 /bin/bash
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.
This utility manages the system log files, rotating,
compressing, deleting, and/or e-mailing them, as appropriate.
This keeps the /var/log
from getting cluttered with old log files.
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.
Stefano Falsetto has created rottlog, which he considers to be an improved version of logrotate.
Process
Statistics: lists currently
executing processes by owner and PID (process ID). This
is usually invoked with ax or
aux options,
and may be piped to grep
or sed to search for a
specific process (see Example 15.14, “Forcing a log-off” and Example 29.3, “Finding the process associated with a PID”).
bash$ps ax | grep sendmail295 ? S 0:00 sendmail: accepting connections on port 25
To display system processes in graphical “tree” format: ps afjx or ps ax --forest.
Combining the ps command with grep or kill.
bash$ps a | grep mingetty2212 tty2 Ss+ 0:00 /sbin/mingetty tty2 2213 tty3 Ss+ 0:00 /sbin/mingetty tty3 2214 tty4 Ss+ 0:00 /sbin/mingetty tty4 2215 tty5 Ss+ 0:00 /sbin/mingetty tty5 2216 tty6 Ss+ 0:00 /sbin/mingetty tty6 4849 pts/2 S+ 0:00 grep mingettybash$pgrep mingetty2212 mingetty 2213 mingetty 2214 mingetty 2215 mingetty 2216 mingetty
Compare the action of pkill with killall.
Lists currently executing processes in
“tree” format. The -p option
shows the PIDs, as well as the process names.
Continuously updated display of most cpu-intensive
processes. The -b option displays in text
mode, so that the output may be parsed or accessed from
a script.
bash$top -b8:30pm 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 ...
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 and snice, which change the priority of a running process or processes, and skill, which sends a kill signal to a process or processes.
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 xclock880
Example 17.6. 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` " or ... # This entire script could be replaced by # kill $(pidof -x process_name) # or # killall process_name # but it would not be as instructive. exit 0
Identifies the processes (by PID) that are accessing
a given file, set of files, or directory. May also be
invoked with the -k option, which kills
those processes. This has interesting implications for
system security, especially in scripts preventing
unauthorized users from accessing system services.
bash$fuser -u /usr/bin/vim/usr/bin/vim: 3207e(bozo)bash$fuser -u /dev/null/dev/null: 3009(bozo) 3010(bozo) 3197(bozo) 3199(bozo)
One important application for fuser is when physically inserting or removing storage media, such as CD ROM disks or USB flash drives. Sometimes trying a umount fails with a device is busy error message. This means that some user(s) and/or process(es) are accessing the device. An fuser -um /dev/device_name will clear up the mystery, so you can kill any relevant processes.
bash$umount /mnt/usbdriveumount: /mnt/usbdrive: device is busybash$fuser -um /dev/usbdrive/mnt/usbdrive: 1772c(bozo)bash$kill -9 1772bash$umount /mnt/usbdrive
The fuser command, invoked with the
-n option identifies the processes
accessing a port. This
is especially useful in combination with nmap.
root#nmap localhost.localdomainPORT STATE SERVICE 25/tcp open smtproot#fuser -un tcp 2525/tcp: 2095(root)root#ps ax | grep 2095 | grep -v grep2095 ? Ss 0:00 sendmail: accepting connections
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.
Some flavors of Linux run crond, Matthew Dillon's version of cron.
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.
Command set to shut the system down, usually just prior to a power down.
On some Linux distros, the halt command has 755 permissions, so it can be invoked by a non-root user. A careless halt in a terminal or a script may shut down the system!
Starts or stops a system service.
The startup scripts in /etc/init.d
and /etc/rc.d use this
command to start services at bootup.
root#/sbin/service iptables stopFlushing firewall rules: [ OK ] Setting chains to policy ACCEPT: filter [ OK ] Unloading iptables modules: [ OK ]
Network mapper and port scanner. This command scans a server to locate open ports and the services associated with those ports. It can also report information about packet filters and firewalls. This is an important security tool for locking down a network against hacking attempts.
#!/bin/bash SERVER=$HOST # localhost.localdomain (127.0.0.1). PORT_NUMBER=25 # SMTP port. nmap $SERVER | grep -w "$PORT_NUMBER" # Is that particular port open? # grep -w matches whole words only, #+ so this wouldn't match port 1025, for example. exit 0 # 25/tcp open smtp
Network interface configuration and tuning utility.
bash$ifconfig -alo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0 UP LOOPBACK RUNNING MTU:16436 Metric:1 RX packets:10 errors:0 dropped:0 overruns:0 frame:0 TX packets:10 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:700 (700.0 b) TX bytes:700 (700.0 b)
The ifconfig command 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 $(/sbin/ifconfig | sed -e 's/ .*//')
# Thanks, S.C., for additional comments.Show current network statistics and information,
such as routing tables and active connections. This utility
accesses information in /proc/net
(Chapter 29, /dev and /proc). See Example 29.4, “On-line connect status”.
netstat -r is equivalent to route.
bash$netstatActive Internet connections (w/o servers) Proto Recv-Q Send-Q Local Address Foreign Address State Active UNIX domain sockets (w/o servers) Proto RefCnt Flags Type State I-Node Path unix 11 [ ] DGRAM 906 /dev/log unix 3 [ ] STREAM CONNECTED 4514 /tmp/.X11-unix/X0 unix 3 [ ] STREAM CONNECTED 4513 . . .
A netstat -lptu shows sockets that are listening to ports, and the associated processes. This can be useful for determining whether a computer has been hacked or compromised.
This is the command set for configuring a wireless network. It is the wireless equivalent of ifconfig, above.
General purpose utility for setting up, changing, and analyzing IP (Internet Protocol) networks and attached devices. This command is part of the iproute2 package.
bash$ip link show1: lo: <LOOPBACK,UP> mtu 16436 qdisc noqueue link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00 2: eth0: <BROADCAST,MULTICAST> mtu 1500 qdisc pfifo_fast qlen 1000 link/ether 00:d0:59:ce:af:da brd ff:ff:ff:ff:ff:ff 3: sit0: <NOARP> mtu 1480 qdisc noop link/sit 0.0.0.0 brd 0.0.0.0bash$ip route list169.254.0.0/16 dev lo scope link
Or, in a script:
#!/bin/bash # Script by Juan Nicolas Ruiz # Used with his kind permission. # Setting up (and stopping) a GRE tunnel. # --- start-tunnel.sh --- LOCAL_IP="192.168.1.17" REMOTE_IP="10.0.5.33" OTHER_IFACE="192.168.0.100" REMOTE_NET="192.168.3.0/24" /sbin/ip tunnel add netb mode gre remote $REMOTE_IP \ local $LOCAL_IP ttl 255 /sbin/ip addr add $OTHER_IFACE dev netb /sbin/ip link set netb up /sbin/ip route add $REMOTE_NET dev netb exit 0 ############################################# # --- stop-tunnel.sh --- REMOTE_NET="192.168.3.0/24" /sbin/ip route del $REMOTE_NET dev netb /sbin/ip link set netb down /sbin/ip tunnel del netb exit 0
Show info about or make changes to the kernel routing table.
bash$routeDestination 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
The iptables command set is a packet filtering tool used mainly for such security purposes as setting up network firewalls. This is a complex tool, and a detailed explanation of its use is beyond the scope of this document. Oskar Andreasson's tutorial is a reasonable starting point.
See also shutting down iptables and Example 30.2, “IP addresses”.
Check network and system configuration. This command
lists and
manages the network and system 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$chkconfig --listatd 0:off 1:off 2:off 3:on 4:on 5:on 6:off rwhod 0:off 1:off 2:off 3:off 4:off 5:off 6:off ...
Network packet “sniffer.” This is a tool for analyzing and troubleshooting traffic on a network by dumping packet headers that match specified criteria.
Dump ip packet traffic between hosts bozoville and caduceus:
bash$tcpdump ip host bozoville and caduceus
Of course, the output of tcpdump can be parsed with 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.
mount -t iso9660 /dev/cdrom /mnt/cdrom # Mounts CD ROM. ISO 9660 is a standard CD ROM filesystem. mount /mnt/cdrom # Shortcut, if /mnt/cdrom listed in /etc/fstab
The versatile mount 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 filesystem image before burning it onto a CDR. [90]
Example 17.7. 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.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 “multispindle” laptops with swappable floppy and optical drives, this can cause problems, however.
The newer Linux distros have deprecated
mount and umount.
The successor, for command-line mounting of removable storage
devices, is gnome-mount. It can take the
-d option to mount a device file by its listing in
/dev.
For example, to mount a USB flash drive:
bash$gnome-mount -d /dev/sda1gnome-mount 0.4bash$df. . . /dev/sda1 63584 12034 51550 19% /media/disk
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 16.61, “Securely deleting a file”) or when the lights begin to flicker.
Sets up and configures loopback devices.
Example 17.8. Creating a filesystem in a file
SIZE=1000000 # 1 meg head -c $SIZE < /dev/zero > file # Set up file of designated size. losetup /dev/loop0 file # Set it up as loopback device. mke2fs /dev/loop0 # Create filesystem. mount -o loop /dev/loop0 /mnt # Mount it. # Thanks, S.C.
Creates a swap partition or file. The swap area must subsequently be enabled with swapon.
Enable / disable swap partitition or file. These commands usually take effect at bootup and shutdown.
Create a Linux ext2 filesystem. This command must be invoked as root.
Example 17.9. Adding a new hard drive
#!/bin/bash # Adding a second hard drive to system. # Software configuration. Assumes hardware already mounted. # From an article by the author of the ABS Guide. # In issue #38 of _Linux Gazette_, http://www.linuxgazette.com. ROOT_UID=0 # This script must be run as root. E_NOTROOT=67 # Non-root exit error. if [ "$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
See also Example 17.8, “Creating a filesystem in a file” and Example 31.3, “Creating a ramdisk”.
Create a DOS FAT filesystem.
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.
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. [91] 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.
The lsusb command lists all USB (Universal Serial Bus) buses and the devices hooked up to them.
The usbmodules command outputs information about the driver modules for connected USB devices.
bash$lsusbBus 001 Device 001: ID 0000:0000 Device Descriptor: bLength 18 bDescriptorType 1 bcdUSB 1.00 bDeviceClass 9 Hub bDeviceSubClass 0 bDeviceProtocol 0 bMaxPacketSize0 8 idVendor 0x0000 idProduct 0x0000 . . .
Lists pci busses present.
bash$lspci00:00.0 Host bridge: Intel Corporation 82845 845 (Brookdale) Chipset Host Bridge (rev 04) 00:01.0 PCI bridge: Intel Corporation 82845 845 (Brookdale) Chipset AGP Bridge (rev 04) 00:1d.0 USB Controller: Intel Corporation 82801CA/CAM USB (Hub #1) (rev 02) 00:1d.1 USB Controller: Intel Corporation 82801CA/CAM USB (Hub #2) (rev 02) 00:1d.2 USB Controller: Intel Corporation 82801CA/CAM USB (Hub #3) (rev 02) 00:1e.0 PCI bridge: Intel Corporation 82801 Mobile PCI Bridge (rev 42) . . .
Creates a boot floppy which can be used to bring up the
system if, for example, the MBR (master boot record) becomes
corrupted. Of special interest is the --iso
option, which uses mkisofs to create a
bootable ISO9660 filesystem image
suitable for burning a bootable CDR.
The mkbootdisk command is actually
a Bash script, written by Erik Troan, in the /sbin directory.
Creates an ISO9660 filesystem suitable for a CDR image.
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 (www.procmail.org). It creates a lock file, a semaphore 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 process (and is therefore “busy”). The presence of a lock file permits only restricted access (or no access) to other processes.
lockfile /home/bozo/lockfiles/$0.lock
# Creates a write-protected lockfile prefixed with the name of the script.
lockfile /home/bozo/lockfiles/${0##*/}.lock
# A safer version of the above, as pointed out by E. Choroba.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 Firefox 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 to 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.
[92]
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 #+ Prevent other programs & scripts
# from accessing files/resources used by xyzip.
...Much less useful than the lockfile command is flock. It sets an “advisory” lock on a file and then executes a command while the lock is on. This is to prevent any other process from setting a lock on that file until completion of the specified command.
flock $0 cat $0 > lockfile__$0 # Set a lock on the script the above line appears in, #+ while listing the script to stdout.
Unlike lockfile, flock does not automatically create a lock file.
Creates block or character device files (may be necessary when installing new hardware on the system). The MAKEDEV utility has virtually all of the functionality of mknod, and is easier to use.
Utility for creating device files. It must be run as
root, and in the /dev directory. It is a sort
of advanced version of mknod.
Automatically deletes files which have not been accessed within a specified period of time. Usually invoked by cron to remove stale log files.
The dump command is an elaborate filesystem backup utility, generally used on larger installations and networks. [93] 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
(/dev/fd0*).
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).
[94]
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 Appendix H, Important Files).
Judicious use of ulimit can protect a system against the dreaded fork bomb.
#!/bin/bash
# This script is for illustrative purposes only.
# Run it at your own peril -- it WILL freeze your system.
while true # Endless loop.
do
$0 & # This script invokes itself . . .
#+ forks an infinite number of times . . .
#+ until the system freezes up because all resources exhausted.
done # This is the notorious “sorcerer's appentice” scenario.
exit 0 # Will not exit here, because this script will never terminate.
A ulimit -Hu XX (where
XX is the user process limit) in
/etc/profile would abort
this script when it exceeded the preset limit.
Display user or group disk quotas.
Set user or group disk quotas from the command-line.
User file creation permissions
mask. Limit the default file
attributes for a particular user. All files created
by that user take on the attributes specified by
umask. The (octal) value passed to
umask defines the file permissions
disabled. For example, umask
022 ensures that new files will have at most
755 permissions (777 NAND 022).
[95]
Of course, the user may later change the
attributes of particular files with chmod. The usual practice
is to set the value of umask
in /etc/profile and/or
~/.bash_profile (see Appendix H, Important Files).
Example 17.10. Using umask to hide an output file from prying eyes
#!/bin/bash
# rot13a.sh: Same as "rot13.sh" script, but writes output to "secure" file.
# Usage: ./rot13a.sh filename
# or ./rot13a.sh <filename
# or ./rot13a.sh and supply keyboard input (stdin)
umask 177 # File creation mask.
# Files created by this script
#+ will have 600 permissions.
OUTFILE=decrypted.txt # Results output to file "decrypted.txt"
#+ which can only be read/written
# by invoker of script (or root).
cat "$@" | tr 'a-zA-Z' 'n-za-mN-ZA-M' > $OUTFILE
# ^^ Input from stdin or a file. ^^^^^^^^^^ Output redirected to file.
exit 0
Get info about or make changes to root device, swap space, or video mode. The functionality of rdev has generally been taken over by lilo, but rdev remains useful for setting up a ram disk. This is a dangerous command, if misused.
List installed kernel modules.
bash$lsmodModule 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 a startup script.
Output information about a loadable module.
bash$modinfo hidfilename: /lib/modules/2.4.20-6/kernel/drivers/usb/hid.o description: "USB HID support drivers" author: "Andreas Gal, Vojtech Pavlik <vojtech@suse.cz>" license: "GPL"
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.
[96]
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.
Or even ...
#!/bin/env bash # Queries the $PATH enviromental variable for the location of bash. # Therefore ... # This script will run where Bash is not in its usual place, in /bin. ...
Show shared lib dependencies for an executable file.
bash$ldd /bin/lslibc.so.6 => /lib/libc.so.6 (0x4000c000) /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x80000000)
Run a command repeatedly, at specified time intervals.
The default is two-second intervals, but this may be changed
with the -n option.
watch -n 5 tail /var/log/messages # 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.
Command-line tool for manipulating the root window of the screen.
Example 17.11. Backlight: changes the brightness of the (laptop) screen backlight
#!/bin/bash
# backlight.sh
# reldate 02dec2011
# A bug in Fedora Core 16/17 messes up the keyboard backlight controls.
# This script is a quick-n-dirty workaround, essentially a shell wrapper
#+ for xrandr. It gives more control than on-screen sliders and widgets.
OUTPUT=$(xrandr | grep LV | awk '{print $1}') # Get display name!
INCR=.05 # For finer-grained control, set INCR to .03 or .02.
old_brightness=$(xrandr --verbose | grep rightness | awk '{ print $2 }')
if [ -z "$1" ]
then
bright=1 # If no command-line arg, set brightness to 1.0 (default).
else
if [ "$1" = "+" ]
then
bright=$(echo "scale=2; $old_brightness + $INCR" | bc) # +.05
else
if [ "$1" = "-" ]
then
bright=$(echo "scale=2; $old_brightness - $INCR" | bc) # -.05
else
if [ "$1" = "#" ] # Echoes current brightness; does not change it.
then
bright=$old_brightness
else
if [[ "$1" = "h" || "$1" = "H" ]]
then
echo
echo "Usage:"
echo "$0 [No args] Sets/resets brightness to default (1.0)."
echo "$0 + Increments brightness by 0.5."
echo "$0 - Decrements brightness by 0.5."
echo "$0 # Echoes current brightness without changing it."
echo "$0 N (number) Sets brightness to N (useful range .7 - 1.2)."
echo "$0 h [H] Echoes this help message."
echo "$0 any-other Gives xrandr usage message."
bright=$old_brightness
else
bright="$1"
fi
fi
fi
fi
fi
xrandr --output "$OUTPUT" --brightness "$bright" # See xrandr manpage.
# As root!
E_CHANGE0=$?
echo "Current brightness = $bright"
exit $E_CHANGE0
# =========== Or, alternately . . . ==================== #
#!/bin/bash
# backlight2.sh
# reldate 20jun2012
# A bug in Fedora Core 16/17 messes up the keyboard backlight controls.
# This is a quick-n-dirty workaround, an alternate to backlight.sh.
target_dir=\
/sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/backlight/acpi_video0
# Hardware directory.
actual_brightness=$(cat $target_dir/actual_brightness)
max_brightness=$(cat $target_dir/max_brightness)
Brightness=$target_dir/brightness
let "req_brightness = actual_brightness" # Requested brightness.
if [ "$1" = "-" ]
then # Decrement brightness 1 notch.
let "req_brightness = $actual_brightness - 1"
else
if [ "$1" = "+" ]
then # Increment brightness 1 notch.
let "req_brightness = $actual_brightness + 1"
fi
fi
if [ $req_brightness -gt $max_brightness ]
then
req_brightness=$max_brightness
fi # Do not exceed max. hardware design brightness.
echo
echo "Old brightness = $actual_brightness"
echo "Max brightness = $max_brightness"
echo "Requested brightness = $req_brightness"
echo
# =====================================
echo $req_brightness > $Brightness
# Must be root for this to take effect.
E_CHANGE1=$? # Successful?
# =====================================
if [ "$?" -eq 0 ]
then
echo "Changed brightness!"
else
echo "Failed to change brightness!"
fi
act_brightness=$(cat $Brightness)
echo "Actual brightness = $act_brightness"
scale0=2
sf=100 # Scale factor.
pct=$(echo "scale=$scale0; $act_brightness / $max_brightness * $sf" | bc)
echo "Percentage brightness = $pct%"
exit $E_CHANGE1
Remote distribution client: synchronizes, clones, or backs up a file system on a remote server.
[88] This is the case on a Linux machine or a UNIX system with disk quotas.
[89] The userdel command will fail if the particular user being deleted is still logged on.
[90] For more detail on burning CDRs, see Alex Withers' article, Creating CDs, in the October, 1999 issue of Linux Journal.
[92] Since only root
has write permission in the /var/lock directory,
a user script cannot set a lock file there.
[93] Operators of single-user Linux systems generally prefer something simpler for backups, such as tar.
[94] As of the version
4 update of Bash, the -f
and -c options take a block size
of 512 when in POSIX
mode. Additionally, there are two new options:
-b for socket buffer size, and
-T for the limit on the number of
threads.
[95] NAND is the logical not-and operator. Its effect is somewhat similar to subtraction.
[96] 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.