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New page: BASH supports one type of variable: a string. = Creating or Assigning a Variable = To create a variable or change the value assigned a value to a variable, write the variable name, a...
[[BASH]] supports one type of variable: a string.
= Creating or Assigning a Variable =
To create a variable or change the value assigned a value to a variable, write the variable name, an equal sign, and the value, with no space:
X=5
COLOUR="Red"
NAME="Jason Smith"
EMAIL="jsmith@example.com>
Variable names must start with a letter and contain only letters, underscores, and digits. Variable names are case-sensitive, and UPPERCASE is often used to make it easy to distinguish between variable names and commands and arguments.
= Reading a Variable =
The <code>read</code> command will read a line of input from stdin and assign it to a variable:
read INPUT
This example will read a line of input from stdin (usually the keyboard) and assign it to the variable INPUT.
= Seeing All Variables =
The <code>set</code> command will display the value of all current variables:
$ set
BASH=/bin/bash
BASH_ARGC=()
BASH_ARGV=()
BASH_LINENO=()
BASH_SOURCE=()
BASH_VERSINFO=([0]="3" [1]="2" [2]="33" [3]="1" [4]="release" [5]="x86_64-redhat-linux-gnu")
BASH_VERSION='3.2.33(1)-release'
COLORS=/home/chris/.dir_colors.xterm
COLORTERM=gnome-terminal
COLUMNS=80
CVS_RSH=ssh
DBUS_SESSION_BUS_ADDRESS=unix:abstract=/tmp/dbus-fFVokEovDE,guid=4eb682a22a695ccd33b24f0048cad335
DESKTOP_SESSION=default
DESKTOP_STARTUP_ID=
DIRSTACK=()
DISPLAY=:0.0
EUID=500
GDMSESSION=default
GDM_LANG=en_US.UTF-8
GDM_XSERVER_LOCATION=local
GNOME_DESKTOP_SESSION_ID=Default
GNOME_KEYRING_PID=3441
GNOME_KEYRING_SOCKET=/tmp/keyring-jBoBFl/socket
GROUPS=()
GTK_MODULES=gnomebreakpad
GTK_RC_FILES=/etc/gtk/gtkrc:/home/chris/.gtkrc-1.2-gnome2
G_BROKEN_FILENAMES=1
HISTFILE=/home/chris/.bash_history
HISTFILESIZE=1000
HISTSIZE=1000
HOME=/home/chris
HOSTNAME=concord3.proximity.on.ca
HOSTTYPE=x86_64
IFS=$' \t\n'
INPUTRC=/etc/inputrc
KDEDIRS=/usr
KDE_IS_PRELINKED=1
LANG=en_US.UTF-8
LESSOPEN='|/usr/bin/lesspipe.sh %s'
LINES=24
LOGNAME=chris
LS_COLORS='no=00:fi=00:di=00;34:ln=00;36:pi=40;33:so=00;35:bd=40;33;01:cd=40;33;01:or=01;05;37;41:mi=01;05;
37;41:ex=00;32:*.cmd=00;32:*.exe=00;32:*.com=00;32:*.btm=00;32:*.bat=00;32:*.sh=00;32:*.csh=00;32:*.tar=00;
31:*.tgz=00;31:*.arj=00;31:*.taz=00;31:*.lzh=00;31:*.zip=00;31:*.z=00;31:*.Z=00;31:*.gz=00;31:*.bz2=00;
31:*.bz=00;31:*.tz=00;31:*.rpm=00;31:*.cpio=00;31:*.jpg=00;35:*.gif=00;35:*.bmp=00;35:*.xbm=00;
35:*.xpm=00;35:*.png=00;35:*.tif=00;35:*.doc=01;37;45:'
MACHTYPE=x86_64-redhat-linux-gnu
MAIL=/var/spool/mail/chris
MAILCHECK=60
OPTERR=1
OPTIND=1
OSTYPE=linux-gnu
PATH=/usr/kerberos/bin:/usr/lib64/ccache:/usr/local/bin:/usr/bin:/bin:/usr/X11R6/bin:/home/chris/bin
PIPESTATUS=([0]="0" [1]="0")
PPID=21127
PROMPT_COMMAND='echo -ne "\033]0;${USER}@${HOSTNAME%%.*}:${PWD/#$HOME/~}"; echo -ne "\007"'
PS1='[\u@\h \W]\$ '
PS2='> '
PS4='+ '
PWD=/home/chris
SDL_AUDIODRIVER=esd
SESSION_MANAGER=local/unix:@/tmp/.ICE-unix/3442,unix/unix:/tmp/.ICE-unix/3442
SHELL=/bin/bash
SHELLOPTS=braceexpand:emacs:hashall:histexpand:history:interactive-comments:monitor
SHLVL=2
SSH_AGENT_PID=3503
SSH_ASKPASS=/usr/libexec/openssh/gnome-ssh-askpass
SSH_AUTH_SOCK=/tmp/ssh-JdPTTt3442/agent.3442
TERM=xterm
UID=500
USER=chris
VIRSH_DEFAULT_CONNECT_URI=qemu:///system
WINDOWID=102760527
WINDOWPATH=7
XAUTHORITY=/tmp/.gdmYDNCHU
XDG_DATA_DIRS=/usr/local/share/:/usr/share/:/usr/share/gdm/
XDG_SESSION_COOKIE=8d5bac776a73b1f9ff24d500473f1f00-1221251892.355110-634173433
_=
colors=/home/chris/.dir_colors.xterm
consoletype=pty
= Using a Variable's Value =
To use the value of a variable in a command, place a dollar sign in front of the variable name:
echo "$COLOUR"
mail -s "Today's colour is $COLOUR" "$NAME <$EMAIL>" </tmp/message
if [ "$X" -eq 5 ]
then
echo "Hooray!"
fi
= Exporting Variables =
Shell variables may be turned into environment variables, which causes them to be inherited by subprocesses:
$ TEST="Yes"
$ bash -c 'echo $TEST'
<nowiki> </nowiki>
$ export TEST
$ bash -c 'echo $TEST'
Yes
$
= Destroying Variables =
A variable will be destroyed when the shell in which it was created exits. You can destroy it using the <code>unset</code> command:
$ Z=100
$ echo $Z
100
$ unset Z
$ echo $Z
<nowiki> </nowiki>
$
= Special Variables =
BASH automatically updates the value of certain special variables:
{|class="wikimedia sortable" border="2"
|-
!Variable
!Description
|-
|$?
|[[BASH Exit Status|Exit status of last pipeline]]
|-
|$$
|Process ID of the current shell
|-
|$!
|Process ID of the last background pipeline
|-
|$RANDOM
|Random integer
|}
= Common Environment Variables =
Linux systems use a number of common environment variables:
{|class="wikimedia sortable" border="2"
|-
!Variable
!Description
|-
|$PATH
|List of colon-separated directories to be used when searching for a command.
|-
|$HOME
|Current user's home directory.
|-
|$MAIL
|Current user's mailbox.
|-
|$DISPLAY
|X window display specification.
|-
|$TERM
|Current terminal type (used to analyze keypresses and send special codes such as colours and effects to the terminal).
|-
|SHELL
|Absolute pathname of the default shell for the current user.
|-
|HOSTNAME
|Name of the host (computer) on which the shell is executing.
|-
|PS1
|Primary prompt, used by the shell to request a command from the user.
|-
|PS2
|Secondary prompt, used to request additional info from the user.
|-
|PS3
|3rd prompt (rarely used).
|-
|PS4
|4th prompt (rarely used).
|}
= Creating or Assigning a Variable =
To create a variable or change the value assigned a value to a variable, write the variable name, an equal sign, and the value, with no space:
X=5
COLOUR="Red"
NAME="Jason Smith"
EMAIL="jsmith@example.com>
Variable names must start with a letter and contain only letters, underscores, and digits. Variable names are case-sensitive, and UPPERCASE is often used to make it easy to distinguish between variable names and commands and arguments.
= Reading a Variable =
The <code>read</code> command will read a line of input from stdin and assign it to a variable:
read INPUT
This example will read a line of input from stdin (usually the keyboard) and assign it to the variable INPUT.
= Seeing All Variables =
The <code>set</code> command will display the value of all current variables:
$ set
BASH=/bin/bash
BASH_ARGC=()
BASH_ARGV=()
BASH_LINENO=()
BASH_SOURCE=()
BASH_VERSINFO=([0]="3" [1]="2" [2]="33" [3]="1" [4]="release" [5]="x86_64-redhat-linux-gnu")
BASH_VERSION='3.2.33(1)-release'
COLORS=/home/chris/.dir_colors.xterm
COLORTERM=gnome-terminal
COLUMNS=80
CVS_RSH=ssh
DBUS_SESSION_BUS_ADDRESS=unix:abstract=/tmp/dbus-fFVokEovDE,guid=4eb682a22a695ccd33b24f0048cad335
DESKTOP_SESSION=default
DESKTOP_STARTUP_ID=
DIRSTACK=()
DISPLAY=:0.0
EUID=500
GDMSESSION=default
GDM_LANG=en_US.UTF-8
GDM_XSERVER_LOCATION=local
GNOME_DESKTOP_SESSION_ID=Default
GNOME_KEYRING_PID=3441
GNOME_KEYRING_SOCKET=/tmp/keyring-jBoBFl/socket
GROUPS=()
GTK_MODULES=gnomebreakpad
GTK_RC_FILES=/etc/gtk/gtkrc:/home/chris/.gtkrc-1.2-gnome2
G_BROKEN_FILENAMES=1
HISTFILE=/home/chris/.bash_history
HISTFILESIZE=1000
HISTSIZE=1000
HOME=/home/chris
HOSTNAME=concord3.proximity.on.ca
HOSTTYPE=x86_64
IFS=$' \t\n'
INPUTRC=/etc/inputrc
KDEDIRS=/usr
KDE_IS_PRELINKED=1
LANG=en_US.UTF-8
LESSOPEN='|/usr/bin/lesspipe.sh %s'
LINES=24
LOGNAME=chris
LS_COLORS='no=00:fi=00:di=00;34:ln=00;36:pi=40;33:so=00;35:bd=40;33;01:cd=40;33;01:or=01;05;37;41:mi=01;05;
37;41:ex=00;32:*.cmd=00;32:*.exe=00;32:*.com=00;32:*.btm=00;32:*.bat=00;32:*.sh=00;32:*.csh=00;32:*.tar=00;
31:*.tgz=00;31:*.arj=00;31:*.taz=00;31:*.lzh=00;31:*.zip=00;31:*.z=00;31:*.Z=00;31:*.gz=00;31:*.bz2=00;
31:*.bz=00;31:*.tz=00;31:*.rpm=00;31:*.cpio=00;31:*.jpg=00;35:*.gif=00;35:*.bmp=00;35:*.xbm=00;
35:*.xpm=00;35:*.png=00;35:*.tif=00;35:*.doc=01;37;45:'
MACHTYPE=x86_64-redhat-linux-gnu
MAIL=/var/spool/mail/chris
MAILCHECK=60
OPTERR=1
OPTIND=1
OSTYPE=linux-gnu
PATH=/usr/kerberos/bin:/usr/lib64/ccache:/usr/local/bin:/usr/bin:/bin:/usr/X11R6/bin:/home/chris/bin
PIPESTATUS=([0]="0" [1]="0")
PPID=21127
PROMPT_COMMAND='echo -ne "\033]0;${USER}@${HOSTNAME%%.*}:${PWD/#$HOME/~}"; echo -ne "\007"'
PS1='[\u@\h \W]\$ '
PS2='> '
PS4='+ '
PWD=/home/chris
SDL_AUDIODRIVER=esd
SESSION_MANAGER=local/unix:@/tmp/.ICE-unix/3442,unix/unix:/tmp/.ICE-unix/3442
SHELL=/bin/bash
SHELLOPTS=braceexpand:emacs:hashall:histexpand:history:interactive-comments:monitor
SHLVL=2
SSH_AGENT_PID=3503
SSH_ASKPASS=/usr/libexec/openssh/gnome-ssh-askpass
SSH_AUTH_SOCK=/tmp/ssh-JdPTTt3442/agent.3442
TERM=xterm
UID=500
USER=chris
VIRSH_DEFAULT_CONNECT_URI=qemu:///system
WINDOWID=102760527
WINDOWPATH=7
XAUTHORITY=/tmp/.gdmYDNCHU
XDG_DATA_DIRS=/usr/local/share/:/usr/share/:/usr/share/gdm/
XDG_SESSION_COOKIE=8d5bac776a73b1f9ff24d500473f1f00-1221251892.355110-634173433
_=
colors=/home/chris/.dir_colors.xterm
consoletype=pty
= Using a Variable's Value =
To use the value of a variable in a command, place a dollar sign in front of the variable name:
echo "$COLOUR"
mail -s "Today's colour is $COLOUR" "$NAME <$EMAIL>" </tmp/message
if [ "$X" -eq 5 ]
then
echo "Hooray!"
fi
= Exporting Variables =
Shell variables may be turned into environment variables, which causes them to be inherited by subprocesses:
$ TEST="Yes"
$ bash -c 'echo $TEST'
<nowiki> </nowiki>
$ export TEST
$ bash -c 'echo $TEST'
Yes
$
= Destroying Variables =
A variable will be destroyed when the shell in which it was created exits. You can destroy it using the <code>unset</code> command:
$ Z=100
$ echo $Z
100
$ unset Z
$ echo $Z
<nowiki> </nowiki>
$
= Special Variables =
BASH automatically updates the value of certain special variables:
{|class="wikimedia sortable" border="2"
|-
!Variable
!Description
|-
|$?
|[[BASH Exit Status|Exit status of last pipeline]]
|-
|$$
|Process ID of the current shell
|-
|$!
|Process ID of the last background pipeline
|-
|$RANDOM
|Random integer
|}
= Common Environment Variables =
Linux systems use a number of common environment variables:
{|class="wikimedia sortable" border="2"
|-
!Variable
!Description
|-
|$PATH
|List of colon-separated directories to be used when searching for a command.
|-
|$HOME
|Current user's home directory.
|-
|Current user's mailbox.
|-
|$DISPLAY
|X window display specification.
|-
|$TERM
|Current terminal type (used to analyze keypresses and send special codes such as colours and effects to the terminal).
|-
|SHELL
|Absolute pathname of the default shell for the current user.
|-
|HOSTNAME
|Name of the host (computer) on which the shell is executing.
|-
|PS1
|Primary prompt, used by the shell to request a command from the user.
|-
|PS2
|Secondary prompt, used to request additional info from the user.
|-
|PS3
|3rd prompt (rarely used).
|-
|PS4
|4th prompt (rarely used).
|}
