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Project Notes

Parameter Substitution

All about Bash parameter substitution.

Notes

The ${} syntax is used to manipulating and/or expanding variables.

Explicit Reference

${parameter}

Reference with Default

${parameter-default}, ${parameter:-default}

If parameter not set, use default. Does not modify the variable. The : form also applies the substitution when the parameter is set but is null.

Reference with Set Default

${parameter=default}, ${parameter:=default}

If parameter not set, set it to default. The : form also applies the substitution when the parameter is set but is null.

Alternate Value

${parameter+alt_value}, ${parameter:+alt_value}

If parameter set, use alt_value, else use null string. The : form also applies the substitution when the parameter is set but is null.

Reference Or Error

${parameter?err_msg}, ${parameter:?err_msg}

If parameter set, use it, else print err_msg and abort the script with an exit status of 1 The : form also applies the substitution when the parameter is set but is null.

Variable length

${#var}

String length (number of characters in $var).

For an array:

  • ${#array} is the length of the first element in the array
  • ${#array[*]} and ${#array[@]} give the number of elements in the array

Remove Leading Match

${var#Pattern} Remove from $var the shortest part of $Pattern that matches the front end of $var.

${var##Pattern} Remove from $var the longest part of $Pattern that matches the front end of $var.

Remove Trailing Match

${var%Pattern} Remove from $var the shortest part of $Pattern that matches the back end of $var.

${var%%Pattern} Remove from $var the longest part of $Pattern that matches the back end of $var.

Variable expansion

${var:pos} Variable var expanded, starting from offset pos.

${var:pos:len} Expansion to a max of len characters of variable var, from offset pos

Substring replacement

${var/Pattern/Replacement} First match of Pattern, within var replaced with Replacement.

${var//Pattern/Replacement} Global replacement. All matches of Pattern, within var replaced with Replacement

${var/#Pattern/Replacement} If prefix of var matches Pattern, then substitute Replacement for Pattern.

${var/%Pattern/Replacement} If suffix of var matches Pattern, then substitute Replacement for Pattern.

Example Script

See example.sh:

$ ./example.sh
## Explicit Reference
Given:
  var1=value1
${var1}: value1
## Reference with Default
Given:
  var2= (null)
  var3= (undefined)
${var1-default}: value1
${var1:-default}: value1
${var2-default}:
${var2:-default}: default
${var3-default}: default
${var3:-default}: default
## Reference with Set Default
Given:
  var4= (null)
  var5= (undefined)
${var4=default}:
${var4}:
${var5=default}: default
${var5}: default
Given:
  var6= (null)
  var7= (undefined)
${var6:=default}: default
${var6}: default
${var7:=default}: default
${var7}: default
## Alternate Value
Given:
  var8=value8
  var9= (null)
  var10= (undefined)
${var8+alt}: alt
${var8:+alt}: alt
${var9+alt}: alt
${var9:+alt}:
${var10+alt}:
${var10:+alt}:
## Variable length
Given:
  var11=this string is 33 characters long
${#var11}: 33
## Remove Leading Match
Given:
  var12=abc/def/ghi/jkl
${var12#*/}: def/ghi/jkl
${var12##*/}: jkl
## Remove Trailing Match
Given:
  var13=abc/def/ghi/jkl
${var13%/*}: abc/def/ghi
${var13%%/*}: abc
## Variable expansion
Given:
  var14=abcdefghijkl
${var14:3}: defghijkl
${var14:3:3}: def
## Substring replacement
Given:
  var15=abcdefghijklm-abcdefghijklm
${var15/def/...}: abc...ghijklm-abcdefghijklm
${var15//def/...}: abc...ghijklm-abc...ghijklm
${var15/#abc/...}: ...defghijklm-abcdefghijklm
${var15/#klm/...}: abcdefghijklm-abcdefghijklm
${var15/%abc/...}: abcdefghijklm-abcdefghijklm
${var15/%klm/...}: abcdefghijklm-abcdefghij...

Credits and References

About LCK#241 bash
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This page is a web-friendly rendering of my project notes shared in the LittleCodingKata GitHub repository.

LittleCodingKata is my collection of programming exercises, research and code toys broadly spanning things that relate to programming and software development (languages, frameworks and tools).

These range from the trivial to the complex and serious. Many are inspired by existing work and I'll note credits and references where applicable. The focus is quite scattered, as I variously work on things new and important in the moment, or go back to revisit things from the past.

This is primarily a personal collection for my own edification and learning, but anyone who stumbles by is welcome to borrow, steal or reference the work here. And if you spot errors or issues I'd really appreciate some feedback - create an issue, send me an email or even send a pull-request.