What are the special dollar sign shell variables?

861
votes

In Bash, there appear to be several variables which hold special, consistently-meaning values. For instance,

./myprogram &; echo $!

will return the PID of the process which backgrounded myprogram. I know of others, such as $? which I think is the current TTY. Are there others?

4
bashenvironment-variablesdollar-sign
Several of them are not Bash-only. They're also used in other Bourne-related shells and in fact are specified by POSIX. - Dennis Williamson
What about: IFS=$'\n' See: stackoverflow.com/questions/4128235/… - sgu
@sgu That's not a parameter; that's a special type of quoting. $'\n' is a literal newline character that result from replacing the digraph \n with ASCII 10. - chepner
If you came here looking for ${1}, ${*}, etc, the braces are just for disambiguation, and often redundant. In isolation, ${x} is exactly equivalent to $x. - tripleee
for $IFS see What is the exact meaning of IFS=$'\n' - Pmpr

4 Answers

1476
votes
  • $1, $2, $3, ... are the positional parameters.
  • "$@" is an array-like construct of all positional parameters, {$1, $2, $3 ...}.
  • "$*" is the IFS expansion of all positional parameters, $1 $2 $3 ....
  • $# is the number of positional parameters.
  • $- current options set for the shell.
  • $$ pid of the current shell (not subshell).
  • $_ most recent parameter (or the abs path of the command to start the current shell immediately after startup).
  • $IFS is the (input) field separator.
  • $? is the most recent foreground pipeline exit status.
  • $! is the PID of the most recent background command.
  • $0 is the name of the shell or shell script.

Most of the above can be found under Special Parameters in the Bash Reference Manual. There are all the environment variables set by the shell.

For a comprehensive index, please see the Reference Manual Variable Index.

43
votes
  • $_ last argument of last command
  • $# number of arguments passed to current script
  • $* / $@ list of arguments passed to script as string / delimited list

off the top of my head. Google for bash special variables.

16
votes

To help understand what do $#, $0 and $1, ..., $n do, I use this script:

#!/bin/bash

for ((i=0; i<=$#; i++)); do
  echo "parameter $i --> ${!i}"
done

Running it returns a representative output:

$ ./myparams.sh "hello" "how are you" "i am fine"
parameter 0 --> myparams.sh
parameter 1 --> hello
parameter 2 --> how are you
parameter 3 --> i am fine
3
votes

Take care with some of the examples; $0 may include some leading path as well as the name of the program. Eg save this two line script as ./mytry.sh and the execute it.

#!/bin/bash

echo "parameter 0 --> $0" ; exit 0

Output:

parameter 0 --> ./mytry.sh

This is on a current (year 2016) version of Bash, via Slackware 14.2