bash parameter substituation

psub.md

cheat sheet for https://tldp.org/LDP/abs/html/parameter-substitution.html

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Variable existence checking

${var:?}          # returns exit-code 1 if var not defined or null
${var:?err_msg}   # prints err_msg and returns exit-code 1 if var not defined or null

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Variable defaulting

${var:-default}   # returns var if defined and non-null, else default

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Variable length

${#var}           # prints length of var to stdout
${#var[@]}        # prints length of array var to stdout

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Substring

${var:pos:len}    # get substring starting at pos (zero-indexed), with length len
${var:pos}        # get substring starting at pos (zero-indexed) until end of string

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Removing prefixes & suffixes from a bash variable

${var#Pattern}    # remove shortest prefix that matches Pattern
${var##Pattern}   # remove longest prefix that matches Pattern
${var%Pattern}    # remove shortest suffix that matches Pattern
${var%%Pattern}   # remove longest suffix that matches Pattern

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Replacing things in a bash variable

${var/Pattern/Replacement}  # replace first occurance of Pattern with Replacement
${var//Pattern/Replacement} # globally replace Pattern with Replacement
${var/#Pattern/Replacement} # replace prefix Pattern with Replacement
${var/%Pattern/Replacement} # replace suffix Pattern with Replacement