Exits the current shell. You can pass an optional exit code, a number. An exit code of 0 (zero) indicates the script executed successfully. A nonzero value indicates an error.

$ exit

Exits the current shell.

None.

Attempts to classify the type of each file passed on the command line. Usually, the file command does this by reading the first few bytes of a file and looking for matches in a file of magic values, /etc/magic. This isn't really magic but simple comparisons. For example, the file command should be able to determine ASCII text files, executable programs, and other types of files.

$ file `which sh`
/bin/sh: symbolic link to `bash'

Prints the type of file of sh. On this system, sh is implemented by bash.

$ file vercompare.py
vercompare.py: a /usr/bin/python script text executable

Checks the type of a Python script.

Sends a signal to the given process or processes. In most cases, the processes die upon receipt of the signal.

$ kill -SIGKILL 4753

Sends the kill signal (SIGKILL) to process number 4753.

$ kill −9 4754

Sends the kill signal (9) to process number 4754.

$ kill -l
 1) SIGHUP       2) SIGINT       3) SIGQUIT      4) SIGILL
 5) SIGTRAP      6) SIGABRT      7) SIGBUS       8) SIGFPE
 9) SIGKILL     10) SIGUSR1     11) SIGSEGV     12) SIGUSR2
13) SIGPIPE     14) SIGALRM     15) SIGTERM     17) SIGCHLD
18) SIGCONT     19) SIGSTOP     20) SIGTSTP     21) SIGTTIN
22) SIGTTOU     23) SIGURG      24) SIGXCPU     25) SIGXFSZ
26) SIGVTALRM   27) SIGPROF     28) SIGWINCH    29) SIGIO
30) SIGPWR      31) SIGSYS      34) SIGRTMIN    35) SIGRTMIN+1
36) SIGRTMIN+2  37) SIGRTMIN+3  38) SIGRTMIN+4  39) SIGRTMIN+5
40) SIGRTMIN+6  41) SIGRTMIN+7  42) SIGRTMIN+8  43) SIGRTMIN+9
44) SIGRTMIN+10 45) SIGRTMIN+11 46) SIGRTMIN+12 47) SIGRTMIN+13
48) SIGRTMIN+14 49) SIGRTMIN+15 50) SIGRTMAX-14 51) SIGRTMAX-13
52) SIGRTMAX-12 53) SIGRTMAX-11 54) SIGRTMAX-10 55) SIGRTMAX-9
56) SIGRTMAX-8  57) SIGRTMAX-7  58) SIGRTMAX-6  59) SIGRTMAX-5
60) SIGRTMAX-4  61) SIGRTMAX-3  62) SIGRTMAX-2  63) SIGRTMAX-1
64) SIGRTMAX

Lists the signals and their numbers.

Displays the online manual entry for the given command.

$ man man
man(1)                                                                 man(1)

NAME
       man - format and display the on-line manual pages

SYNOPSIS
       man  [-acdfFhkKtwW]  [--path] [-m system] [-p string] [-C config_file]
       [-M pathlist] [-P pager] [-S section_list] [section] name ...

DESCRIPTION
       man formats and displays the on-line manual  pages.   If  you  specify
       section,  man  only looks in that section of the manual.  name is nor-
       mally the name of the manual page, which is typically the  name  of  a
       command,  function,  or  file.   However, if name contains a slash (/)
       then man interprets it as a file specification, so that you can do man
       ./foo.5 or even man /cd/foo/bar.1.gz.

       See  below  for  a  description of where man looks for the manual page
       files.
...

Displays help on the man command.

Options differ by platform. Try the man man command to see the options for your platform.

Short for no hangup, the nohup command runs a command and keeps that command running even if you log out. Typically, when you log out, all the commands you launched are terminated if they are still running. The "no hangup" terminology comes from the days when users logged in using a modem over a phone line and would literally hang up the phone when exiting.

$ nohup xclock &
[1] 4833
nohup: appending output to `nohup.out'

Runs the xclock command in the background, preserving the process even if you log out.

None.

Prints out the value of a given environment variable or all environment variables if you pass no arguments to this command.

$ printenv USER
ericfj

Prints the USER environment variable.

None.

Prints the status of current processes, depending on the command-line options. With no options, ps lists just the current shell and the ps process. With options, you can list all the processes running on the system.

Note that Berkeley Unix-based systems, including Mac OS X, support a different set of options than System V Unix-based systems. The options to list all processes is aux for Berkeley Unix-based systems and -ef for System V Unix-based systems. Linux systems support both types of options.

$ ps
  PID TTY          TIME CMD
 4267 pts/2    00:00:00 bash
 4885 pts/2    00:00:00 ps

Lists the current process (the ps command) and its parent shell.

$ ps -ef
UID        PID  PPID  C STIME TTY          TIME CMD
root         1     0  0 09:57 ?        00:00:00 init [5]
root      2046     1  0 08:46 ?        00:00:00 klogd -x
rpc       2067     1  0 08:46 ?        00:00:00 portmap
rpcuser   2087     1  0 08:46 ?        00:00:00 rpc.statd
root      2290     1  0 08:46 ?        00:00:00 /usr/sbin/sshd
root      2340     1  0 08:46 ?        00:00:00 gpm -m /dev/input/mice -t imps2
root      2350     1  0 08:46 ?        00:00:00 crond
xfs       2376     1  0 08:46 ?        00:00:00 xfs -droppriv -daemon
dbus      2414     1  0 08:46 ?        00:00:00 dbus-daemon-1 --system
root      2427     1  0 08:46 ?        00:00:00 cups-config-daemon
...

Lists all processes in System V Unix style (-ef).

$ ps aux
USER       PID %CPU %MEM   VSZ  RSS TTY      STAT START   TIME COMMAND
root         1  0.0  0.0  3488  560 ?        S    09:57   0:00 init [5]
root         2  0.0  0.0     0    0 ?        SN   09:57   0:00 [ksoftirqd/0]

root         3  0.0  0.0     0    0 ?        S<   09:57   0:00 [events/0]
root         4  0.0  0.0     0    0 ?        S<   09:57   0:00 [khelper]
root         5  0.0  0.0     0    0 ?        S<   09:57   0:00 [kacpid]
root        27  0.0  0.0     0    0 ?        S<   09:57   0:00 [kblockd/0]
root        28  0.0  0.0     0    0 ?        S    09:57   0:00 [khubd]
root        37  0.0  0.0     0    0 ?        S    09:57   0:00 [pdflush]
root        38  0.0  0.0     0    0 ?        S    09:57   0:00 [pdflush]
root        40  0.0  0.0     0    0 ?        S<   09:57   0:00 [aio/0]
root        39  0.0  0.0     0    0 ?        S    09:57   0:00 [kswapd0]
root       113  0.0  0.0     0    0 ?        S    09:57   0:00 [kseriod]
root       187  0.0  0.0     0    0 ?        S    08:46   0:00 [kjournald]
root      1014  0.0  0.0  1612  448 ?        S<s  08:46   0:00 udevd
...

Lists all processes in Berkeley Unix style (aux).

Sleeps for a given number of seconds. You can use an m suffix to indicate minutes and an h suffix for hours.

$ sleep 2

Sleeps for two seconds.

$ sleep 3h

Sleeps for three hours.

None.

Determines the type of command, such as a command on disk or a shell built-in command.

$ type sleep
sleep is /bin/sleep

Returns the type of the sleep command.

$ type type
type is a shell builtin

Returns the type of the type command.

$ type -t type
builtin

Returns the type name of the type command.

$ type -p sleep
/bin/sleep

Returns the path to the sleep command.

$ type -t sleep
file

Returns the type name of the sleep command.

$ type -a true
true is a shell builtin
true is /bin/true

Returns information on all instances found of the true command.

Prints information on the system. Short for Unix name.

$ uname -p
powerpc

Lists the processor type.

$ uname
Darwin

Lists the Unix name.

$ uname -o
GNU/Linux

Lists the OS name.

$ uname -s
Linux

Lists the kernel name (OS name, really).

$ uname --hardware-platform
i386

Lists the hardware platform, similar to the processor type.

Shows who is logged on, as well as information about the system.

$ who am i
ericfj   pts/1        Jan 16 15:28 (:0.0)

Lists who the user is.

$ who -b
         system boot  Jan 16 08:46

Lists the last boot time.

$ who
ericfj   :0           Jan 16 15:28
ericfj   pts/1        Jan 16 15:28 (:0.0)
ericfj   pts/2        Jan 16 15:28 (:0.0)
ericfj   pts/3        Jan 16 15:28 (:0.0)

Lists all logged-in users. Note how it thinks the same user is logged in multiple times. The pts 1, 2, and 3 values come from shell windows.

$ who -H
NAME     LINE         TIME         COMMENT
ericfj   :0           Jan 16 15:28
ericfj   pts/1        Jan 16 15:28 (:0.0)
ericfj   pts/2        Jan 16 15:28 (:0.0)
ericfj   pts/3        Jan 16 15:28 (:0.0)

Adds a header line to the normal who output.

$ who -q
ericfj ericfj ericfj ericfj
# users=4

Lists in quick mode.

$ who -r
         run-level 5  Jan 16 08:46                   last=S

Lists the system run level. On Linux, run level 5 usually means the X Window System has been started for graphics.

Prints out the username of the current user.

$ whoami
ericfj

Lists who the user is.

None.

Extracts the base file name from a long path. The optional suffix allows you to extract a file-name extension, such as .txt.

$ basename /home/ericfj/rpms/thunderbird-1.0-1.fc3.i386.rpm
thunderbird-1.0-1.fc3.i386.rpm
$ basename /home/ericfj/rpms/thunderbird-1.0-1.fc3.i386.rpm .rpm
thunderbird-1.0-1.fc3.i386

None.

Concatenates files to standard output. You can concatenate one or more files. With just one file, cat prints the file to standard output, and you can use this to display the contents of short files. With multiple files, cat prints them all to standard output, allowing you to combine files together. You'll often use output redirection such as > or >> with cat.

$ cat /etc/passwd
root:x:0:0:root:/root:/bin/bash
bin:x:1:1:bin:/bin:/sbin/nologin
daemon:x:2:2:daemon:/sbin:/sbin/nologin
adm:x:3:4:adm:/var/adm:/sbin/nologin
lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin
sync:x:5:0:sync:/sbin:/bin/sync
shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown
halt:x:7:0:halt:/sbin:/sbin/halt
...

Shows the contents of the file /etc/password.

$ cat /etc/shells
/bin/sh
/bin/bash
/sbin/nologin
/bin/ash
/bin/bsh
/bin/ksh
/usr/bin/ksh
/usr/bin/pdksh
/bin/tcsh
/bin/csh
/bin/zsh

Shows the contents of the file /etc/shells.

Options differ by platform. See your online documentation for details on your platform.

Changes the mode, the permissions, on a file.

The following table lists the numeric modes for the chmod command. Note that these modes are all in octal, base 8, numbers.

You then need to add these values together, as in the following table.

This example results in a total of 764.

In addition to the numeric modes, you can use the symbolic modes, as shown in the following table.

$ chmod a+x script1

Adds execute permissions for all users to script1.

$ chmod 764 script1

Allows the user to read, write, and execute script1, the members of the group to read and write, and everyone else to just read.

Changes the ownership of files. You must be the owner of the file or files.

$ chown ericfj  script1 script2 script3

Copies a file or files. If you copy multiple files, then the destination must be a directory. If you just copy one file, then the destination can be a file name or a directory.

$ cp * /usr/local/bin

Copies all files in the current directory to /usr/local/bin.

$ cp report.txt report.backup

Copies a file to a backup file.

Short for disk free, df returns the amount of space used and available on all mounted file systems. With no arguments, df lists information for all mounted file systems. You can pass the name of the file systems, either the file system or the mount point, to list information on just those file systems. You can also provide the name of a directory, and df displays information on the file system that contains that directory. This is very handy so you don't have to remember all the file system names.

For example, an all-too-frequent problem occurs when the /tmp, or temporary, directory fills up. On some systems, /tmp is mounted as its own file system (and disk partition). On other systems, /tmp is part of the root, or /, file system. You can pass /tmp as the name of a file system to the df command. Even if /tmp is not mounted as part of its own file system, the df command will display information on the file system holding /tmp.

$ df
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/hda2             24193540   3979392  18985176  18% /
/dev/hda1               101086     10933     84934  12% /boot
none                    501696         0    501696   0% /dev/shm
/dev/hda5             48592392  26391104  19732904  58% /home2

Lists all mounted file systems.

$ df /tmp
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/hda2             24193540   3979392  18985176  18% /

Lists the information for the file system containing /tmp.

Lists the amount of disk space used for a given set of files or directories. Technically, du estimates the amount of disk usage. You can use du on a file or a directory. The command will traverse all subdirectories and report on the total for each directory it examines. Command-line options can modify this behavior.

$ du
360     ./mrtg/html
632     ./mrtg/working
1100    ./mrtg
328     ./marketing
88      ./web_files
18860   ./figures/tmp/chap14
19460   ./figures/tmp
20      ./figures/.xvpics
136     ./figures/chap1
1416    ./figures/chap2
9872    ./figures/chap14_1
8996    ./figures/chap14_2
53508   ./figures
2228    ./scripts/foo
37284   ./scripts
18688   ./chapter12
5076    ./author_review
122364  .

Shows the size of the current directory and all subdirectories.

$ du -s
122364  .

Runs the same command but in silent mode, showing just a total size.

The find command searches the files on disk from a given starting location, start_at, looking for files and directories that match the given conditions and then taking the given actions, such as printing out the file names.

This is a very complex command. It is often used to make backups (finding the list of files modified since the last backup), report on large files (finding the list of files larger than a certain size), or in fascist environments, remove all old files (finding all files older than a given date and then removing them).

In some cases, you'll want to combine the output of find with other commands. But the number of actions available to the find command itself means that you can often just run the command alone.

$ find . -ctime −1 -print
./scripts
./scripts/nohup.out
./583204_appb_efj.doc

Finds all files in the current directory and below that have been modified in the last day. The -print option is not necessary.

$ find $HOME -name 'script*'
./scripting_outline.txt
./scripts
./scripts/script1
./scripts/script3
./scripts/script2
./scripts/script_q
./scripts/script1.sh
./scripts/script_y

./scripts/script5
./scripts/script4
./scripts/script8
./scripts/script6
./scripts/script7
./scripts/script9
./scripts/script10
./scripts/script11
./scripts/script12
./scripts/script13
./scripts/script14
./scripts/script15
./scripts/script16
./scripts/script17
./scripts/script18

Finds all files in the user's home directory (and below) with a name that starts with script.

See the online manual entry on find, as there are a huge number of options that differ by platform.

Searches for text based on a pattern (called a regular expression). The lines of text that match the pattern are printed. You need to tell grep what to look for and which files to examine.

$ grep while *
whilepipe:while read filename; do

Looks for all files in the current directory for the text while. This is found in one file.

$ grep -h while *
while read filename; do

Performs the same search but does not output the names of the files.

$ grep -o while *
whilepipe:while

Performs the same search but outputs only the pattern match.

In addition to these options, you'll find a number of platform-specific options in your online manuals.

Displays the beginning of a text file. By default, head prints out the first ten lines in the file.

$ head /etc/passwd
root:x:0:0:root:/root:/bin/bash
bin:x:1:1:bin:/bin:/sbin/nologin
daemon:x:2:2:daemon:/sbin:/sbin/nologin
adm:x:3:4:adm:/var/adm:/sbin/nologin
lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin
sync:x:5:0:sync:/sbin:/bin/sync
shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown
halt:x:7:0:halt:/sbin:/sbin/halt
mail:x:8:12:mail:/var/spool/mail:/sbin/nologin
news:x:9:13:news:/etc/news:

Lists the first ten lines of /etc/passwd.

$ head −2 /etc/passwd
root:x:0:0:root:/root:/bin/bash
bin:x:1:1:bin:/bin:/sbin/nologin

Lists the first two lines of /etc/passwd.

Lists file names. You can display a long listing or a short listing. This is a surprisingly complex command for such a simple purpose.

$ ls /usr/local
bin  etc  games  include  lib  libexec  man  sbin  share  src

Lists the files in /usr/local.

$ ls -CF /usr/local
bin/  etc/  games/  include/  lib/  libexec/  man/  sbin/  share/  src/

Lists the files in /usr/local with a slash after directory names, an @ for links, and a * for executable files.

$ ls -l /usr/local
total 80
drwxr-xr-x  2 root root 4096 Dec  9 00:00 bin
drwxr-xr-x  2 root root 4096 Aug 12 12:02 etc
drwxr-xr-x  2 root root 4096 Aug 12 12:02 games
drwxr-xr-x  2 root root 4096 Aug 12 12:02 include
drwxr-xr-x  2 root root 4096 Aug 12 12:02 lib
drwxr-xr-x  2 root root 4096 Aug 12 12:02 libexec
drwxr-xr-x  3 root root 4096 Nov 15 20:35 man
drwxr-xr-x  2 root root 4096 Aug 12 12:02 sbin
drwxr-xr-x  4 root root 4096 Nov 15 17:17 share
drwxr-xr-x  2 root root 4096 Aug 12 12:02 src

Presents a long listing of the files in /usr/local.

In addition to these options, you'll find a number of platform-specific options in your online manuals.

Creates one or more directories.

$ mkdir tmp

Creates directory tmp.

$ mkdir -m 664 tmp
$ ls -dl tmp
drw-rw-r--  2 ericfj ericfj 4096 Jan 16 20:58 tmp

Creates directory tmp with the given permissions (verified by the ls command).

Moves a file or files. If you move multiple files, the target must be a directory. If you move one file, the target can be a file name or a directory name (naming the directory in which to move the file).

$ mv *.html old_web

Moves all HTML files to the directory named old_web.

$ mv index.htm index.html

Renames the file index.htm to index.html.

Removes (deletes) files.

$ rm -rf ./tmp

Removes the tmp subdirectory and all files and directories in it.

$ rm -i index.html
rm: remove regular file `index.html'? y

Removes the file named index.html but requires you to confirm the deletion (a smart option).

Removes (deletes) directories. The directories must be empty to be deleted. Watch out for hidden files (files with names that start with a period), because you won't see these files, but their presence will stop rmdir from deleting a directory.

$ rmdir tmp

Removes the subdirectory tmp.

Prints out the last ten lines of a file. You can define the number of lines. The -f option tells the tail command to output forever, checking the file periodically for new lines and then printing those. This is most useful with log files for a service or when building a huge software package.

$ tail /etc/passwd
mailnull:x:47:47::/var/spool/mqueue:/sbin/nologin
smmsp:x:51:51::/var/spool/mqueue:/sbin/nologin
pcap:x:77:77::/var/arpwatch:/sbin/nologin
apache:x:48:48:Apache:/var/www:/sbin/nologin
squid:x:23:23::/var/spool/squid:/sbin/nologin
webalizer:x:67:67:Webalizer:/var/www/usage:/sbin/nologin

xfs:x:43:43:X Font Server:/etc/X11/fs:/sbin/nologin
ntp:x:38:38::/etc/ntp:/sbin/nologin
gdm:x:42:42::/var/gdm:/sbin/nologin
ericfj:x:500:500:Eric Foster-Johnson:/home2/ericfj:/bin/bash

Lists the last ten lines of /etc/passwd.

$ tail −2 /etc/passwd
gdm:x:42:42::/var/gdm:/sbin/nologin
ericfj:x:500:500:Eric Foster-Johnson:/home2/ericfj:/bin/bash

Lists the last two lines of /etc/passwd.

$ tail -f /var/log/dmesg
EXT3 FS on hda1, internal journal
EXT3-fs: mounted filesystem with ordered data mode.
SELinux: initialized (dev hda1, type ext3), uses xattr
SELinux: initialized (dev tmpfs, type tmpfs), uses transition SIDs
kjournald starting.  Commit interval 5 seconds
EXT3 FS on hda5, internal journal
EXT3-fs: mounted filesystem with ordered data mode.
SELinux: initialized (dev hda5, type ext3), uses xattr
Adding 4096564k swap on /dev/hda3.  Priority:-1 extents:1
SELinux: initialized (dev binfmt_misc, type binfmt_misc), uses genfs_contexts

Outputs the contents of the log file dmesg forever (until killed).

By touching a file, you modify it. (Think of what happens when children touch something.) At the most basic level, touch is used to update the time a file was last modified. You can also set the time to a particular value.

Typically, if a file doesn't exist, touch will create it, making it 0 (zero) bytes in size.

To set the time to a particular value, use one of the following formats: CCYYMMddhhmm, YYMMddhhmm, MMddhhmm, or MMddhhmmYY. The following table explains the formats.

$ touch *.c

Updates the modification date for all files ending in .c to the current time.

$ touch -t 201012251159 mozilla_coffee_order.html
$ ls -l mozilla_coffee_order.html
-rw-rw-r--  1 ericfj ericfj 7663 Dec 25  2010 mozilla_coffee_order.html

Sets the modification time to Christmas in 2010 for the given file and verifies the time with the ls command.

The awk command runs a program, typically placed between single quotes (as shown here) or in a separate file. The awk command searches the files passed to it for the pattern /somedata/ and then applies the given actions to all lines matching the pattern.

See Chapter 7 for a lot of information on awk. This is a very complex command.

$  awk -F':' '/eric/ { print $5 }' /etc/passwd
Eric Foster-Johnson

Searches for the pattern eric in the file /etc/passwd and then prints out the fifth field from all matching lines. Sets the field separator to a colon (:) instead of the default spaces because of the format of the /etc/passwd file.

Echoes its data to standard output. If you place the data in double quotes, echo expands variables inside text strings.

$ echo $HOME
/home2/ericfj

Lists the user's home directory.

$ echo "User ${USER}'s home directory is ${HOME}."
User ericfj's home directory is /home2/ericfj.

Lists the user's username and home directory.

$ echo hello    there
hello there

Outputs the two arguments, with one space in between.

$ echo "hello    there"
hello    there

Outputs the same data with the embedded spaces included.

$ echo -n "What is your name? "
What is your name?

Outputs the question and leaves the cursor after the question mark.

Extracts data as columns, based on a field separator, usually a space.

$ cut -d':' -f1,5 /etc/passwd
root:root
bin:bin
daemon:daemon
adm:adm
lp:lp
sync:sync
shutdown:shutdown
halt:halt
mail:mail
news:news
uucp:uucp
operator:operator
games:games
gopher:gopher
ftp:FTP User
nobody:Nobody
dbus:System message bus
vcsa:virtual console memory owner
nscd:NSCD Daemon

rpm:
haldaemon:HAL daemon
netdump:Network Crash Dump user
sshd:Privilege-separated SSH
rpc:Portmapper RPC user
rpcuser:RPC Service User
nfsnobody:Anonymous NFS User
mailnull:
smmsp:
pcap:
apache:Apache
squid:
webalizer:Webalizer
xfs:X Font Server
ntp:
gdm:
ericfj:Eric Foster-Johnson

Cuts the first and fifth fields from the /etc/passwd file.

A stream, or noninteractive, text editor. Use sed to modify files in a programmed manner.

See Chapter 6 for a lot of information on sed.

$ cat /etc/passwd | sed 'p' | head −10
root:x:0:0:root:/root:/bin/bash
root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/bin/sh
daemon:x:1:1:daemon:/usr/sbin:/bin/sh
bin:x:2:2:bin:/bin:/bin/sh
bin:x:2:2:bin:/bin:/bin/sh
sys:x:3:3:sys:/dev:/bin/sh
sys:x:3:3:sys:/dev:/bin/sh
sync:x:4:65534:sync:/bin:/bin/sync
sync:x:4:65534:sync:/bin:/bin/sync

Sends the contents of the /etc/passwd file to the sed command. The 'p' program tells sed to print out the lines. This output is redirected to the head command, which shows only the first ten lines.

Sorts files line by line.

$ printenv | sort | head −4
COLORTERM=gnome-terminal
DBUS_SESSION_BUS_ADDRESS=unix:abstract=/tmp/dbus-LFVLT6j4Fj
DESKTOP_SESSION=default
DISPLAY=:0.0

Sorts the data returned by printenv.

$ printenv | sort | sort -c
$ printenv |  sort -c
sort: -:2: disorder: HOSTNAME=kirkwall

Shows how the -c option works. If the data are sorted, -c tells the sort command to do nothing. Otherwise, it generates an error on the first out-of-order line.

Searches for printable strings in a file or files and then outputs these strings.

$ strings `which awk` | grep opyright
copyright
        -W copyright            --copyright
Copyright (C) 1989, 1991-%d Free Software Foundation.

Searches the awk command for the string opyright (short for copyright).

Translates or deletes characters.

$ cat /etc/passwd | tr ':' ' ' | tail −4
xfs x 43 43 X Font Server /etc/X11/fs /sbin/nologin
ntp x 38 38  /etc/ntp /sbin/nologin
gdm x 42 42  /var/gdm /sbin/nologin
ericfj x 500 500 Eric Foster-Johnson /home2/ericfj /bin/bash

Translates the colon in the /etc/passwd file to a space, for easier reading. Prints the last four lines.

Provides a programmable calculator. The bc command supports its own mini programming language. You can enter commands in the bc language at the command line or pipe the text of the commands to bc.

In bc, the basic data element is a number. You can then use math statements to modify numbers, or you can invoke functions. As a programming language, there are quite a few commands within bc.

$ bc
bc 1.06
Copyright 1991-1994, 1997, 1998, 2000 Free Software Foundation, Inc.
This is free software with ABSOLUTELY NO WARRANTY.
For details type `warranty'.
scale=2
x=10
x + 10
20
x
10
tax=100*7/100
tax
7.00
x = x + tax
x
17.00
print x
17.00
quit

Evaluates an expression, usually a numeric expression. Note that expr works only with whole numbers (integers). For floating-point numbers, use bc.

$ expr 40 + 2
42

Adds 40 plus 2.

$ expr 40 / 10
4

Divides 40 by 10.

$ expr 42 % 10
2

Returns the remainder after dividing 42 by 10.

None.