Shell parameter expansion

Parameter unwinding (parameter expansion) is the procedure that the shell uses to provide variable values in a program.

Expansion operators include substitution operators and pattern matching operators

Substitution operators

Operator	Replacement instructions	Memory
${varname:-word}	If varname exists and is not NULL, its value is returned, otherwise word is returned.	Empty returns Word
${varname:=word}	Returns the value of VarName if it exists and is not NULL, otherwise, sets the value of varname to Word and returns its value. Change the value of the variable.	Empty returns Word and changes the value of the variable
${varname:?message}	If varname exists and is not NULL, its value is returned; otherwise, Varname:message is displayed and the current command or script exits. Omitting the message will show the default information parameter null or not set.	Empty displays a warning and exits.
${varname:+word}	Returns word if varname exists and is not NULL, otherwise NULL is returned.	non-empty returns word, empty returns NULL.

Example:

$ echo ${varname} $ echo ${varname:var1} $ echo ${varname:-var1} Var1 $ echo ${varname:+var1} $ echo ${varname:=var2} Var2 $ echo ${varname:+var1} Var1 $ echo ${varname} Var2 $ echo ${varname:?warning1} Var2 $ varname= $ echo ${varname} $ echo ${VARNAME:+VAR5} $ echo ${varname} $ echo ${varname:?warning2} -bash:varname:warning2

Pattern matching Operators

Operator	Replace	Memory
${variable#Pattern}	If the pattern matches the beginning of the variable value, the shortest part of the match is deleted and the remainder is returned.	Delete the Shortest (one #) part of the beginning
${variable# #Pattern}	If the pattern matches the switch of the variable value, the longest portion of the match is deleted and the remainder is returned.	Delete switch maximum (two #) section
${variable%pattern}	If the pattern matches the end of the variable value, the shortest part of the match is deleted and the rest is returned.	Delete end shortest ( one%) section
${variablepercent pattern}	If the pattern matches the end of the variable value, the longest portion of the match is deleted and the remainder is returned.	Delete End -of-length (two%) section

Example:

$ Export Myvar=/home/merlin/myfile.log.tail $ echo ${myvar} /home/merlin/myfile.log.tail $ echo ${myvar#/*/} Merlin/myfile.log.tail $ echo ${myvar##/*/} Myfile.log.tail $ echo ${myvar%.*} /home/merlin/myfile.log $ echo ${myvar%%.*} /home/merlin/myfile

Shell parameter expansion