Doing variable expansion in Bash, like a boss

29 July 2011

I should have called this, how to kick sed off my living room couch, and tell that lazy scumbag to go out and get a job and stop drinking all my beer. But that was too long.

TL;DR

You can do just about anything inside two {}'s; the laws of nature no longer apply in there..

Substrings

$ export MYVAR=/usr/bin/eric.rocks

$ unset NULLVAR

$ echo ${MYVAR:1}
usr/bin/eric.rocks

$ echo ${MYVAR:9}
eric.rocks

$ echo ${MYVAR:9:4}
eric

# variable name wildcards

$ echo ${!MY*}
MYVAR

$ echo ${!DI*}
DIRSTACK DISPLAY

# '##' removes a prefix (accepts wildcards)

$ echo ${MYVAR##/usr}
/bin/eric.rocks

$ pwd
/usr/bin

$ echo ${MYVAR##$( pwd )}
/eric.rocks

# '%' remove the suffix (non-greedy)

$ echo ${MYVAR%.rocks}
/usr/bin/eric

# '%%' removes it greedily

$ echo ${MYVAR%r*}
/usr/bin/eric.

$ echo ${MYVAR%%r*}
/us

$ echo ${MYVAR%%ic}
/usr/bin/eric.rocks

$ echo ${MYVAR%%ic*}
/usr/bin/er

Substitution

${parameter/pattern/string}

# pattern accepts filename-type wildcards

$ echo ${MYVAR/e/f}
/usr/bin/fric.rocks

$ echo ${MYVAR/e*c/f}
/usr/bin/fks

# '#' means match from beginning

$ echo ${MYVAR/#\/*c/f}
fks

# '//' means replace all instances

$ $ echo ${MYVAR/S/e}                                  
ACCEeS DENIED

$ echo ${MYVAR//S/e}                                 
ACCEee DENIED

Transformations

# '^^' uppercases it

$ echo ${MYVAR^^}
/USR/BIN/ERIC.ROCKS

$ echo ${MYVAR^*}
/usr/bin/eric.rocks

$ echo ${TERM^*}
Screen

# ',' and ',,' lowercases it

$ export MYVAR="ACCESS DENIED"

$ echo ${MYVAR,}

aCCESS DENIED

$ echo ${MYVAR,,} 
access denied

Returning Default Values

$ export MYVAR=/usr/bin/eric.rocks

$ unset NULLVAR

# return default "test" if unset

$ echo ${MYVAR:-test}
/usr/bin/eric.rocks

# return default value if unset

$ echo ${NULLVAR:-test}
test

# return, and assign, default value if unset

$ echo ${NULLVAR:=test}
test

$ set |grep NULLVAR
NULLVAR=test

$ unset NULLVAR

# return error message if unset

$ echo ${NULLVAR?aaaaaah}
-bash: NULLVAR: aaaaaah

# return value if set

$ echo ${MYVAR:+$TERM}
screen

# return nothing if unset

$ echo ${NULLVAR:+$TERM}