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10 Forms of Bash Shell Parameter Expansion

October 15, 2023 #100DaysToOffload #Bash
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If I look at my search history with the word “bash” in it, the most frequently searched phrases turn out to be like “trim suffix bash” and “set bash variable if empty”.

It seems I write Bash scripts frequently enough to need these, but not frequently enough to remember these simple Bash shell parameter expansion forms.

In this blog post I am going to keep a list of 10 forms of Bash shell parameter expansion, that I hope will save me a visit to google.com.

In Bash, the basic form of parameter expansion is ${var}. But there’s more. The official documentation has a detailed list and description of how else you can expand a parameter: https://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html

${var:-fallback}: Use fallback, If var Is Unset 

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A=
B=cookies
C=cakes
echo ${A:-keyboardcat} # ⇒ keyboardcat
echo ${B:-keyboardcat} # ⇒ cookies
echo ${A:-$C}          # ⇒ cakes

${var:+substitute}: Use substitute, If var Is Set 

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A=
B=cookies
C=cakes
echo ${A:+keyboardcat} # ⇒ 
echo ${B:+keyboardcat} # ⇒ keyboard
echo ${C:+$B}          # ⇒ cookies

${#var}: Length of var 

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A="the quick brown fox"
echo ${#A} # ⇒ 19

${var:start}: Substring of var Starting from start 

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A="the quick brown fox"
echo ${A:10} # ⇒ brown fox

${var:start:length}: Substring of var of Size length Starting from start 

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A="the quick brown fox"
echo ${A:4:5} # ⇒ quick

${var#prefix}: Trim prefix From var 

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FILENAME="/home/hjr265/not-important-do-not-open.txt"
DIR="/home/hjr265/"
echo ${FILENAME#/home/hjr265/} # ⇒ not-important-do-not-open.txt
echo ${FILENAME#$DIR}          # ⇒ not-important-do-not-open.txt

${var%suffix}: Trim suffix From var 

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FILENAME="not-important-do-not-open.txt"
EXT=".txt"
echo ${FILENAME%.txt} # ⇒ not-important-do-not-open
echo ${FILENAME%$EXT} # ⇒ not-important-do-not-open

${var/pattern/substitute}: Replace pattern in var with substitute 

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A="the slow brown fox"
B="fox"
C="cow"
echo ${A/slow/quick} # => the quick brown fox
echo ${A/$B/$C}      # => the slow brown cow

${var^}: Change Letter Case in var 

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A="the quick brown fox"
B="THE SLOW BROWN COW"
echo ${A^}  # => The quick brown fox
echo ${A^^} # => THE QUICK BROWN FOX
echo ${B,}  # => tHE SLOW BROWN COW
echo ${B,,} # => the slow brown cow

${var@Q}: Quoted String 

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A="-w -s"
echo ${A@Q} # => '-w -s'

This post is 66th of my #100DaysToOffload challenge. Want to get involved? Find out more at 100daystooffload.com.

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