[bash] string operators

4.3.4. Extended Pattern Matching

Bash provides a further set of pattern matching operators if the shopt option extglob is switched on. Each operator takes one or more patterns, normally strings, separated by the vertical bar ( | ). The extended pattern matching operators are given in Table 4-3.^[8]

^[8] Be aware that these are not available in early releases of bash 2.0.

Table 4-3. Pattern-matching operators

Operator	Meaning
*(patternlist)	Matches zero or more occurrences of the given patterns.
+(patternlist)	Matches one or more occurrences of the given patterns.
?(patternlist)	Matches zero or one occurrences of the given patterns.
@(patternlist)	Matches exactly one of the given patterns.
!(patternlist)	Matches anything except one of the given patterns.

Table 4-2. Pattern-matching operators

Operator	Meaning
${variable#pattern}	If the pattern matches the beginning of the variable's value, delete the shortest part that matches and return the rest.
${variable##pattern}	If the pattern matches the beginning of the variable's value, delete the longest part that matches and return the rest.
${variable%pattern}	If the pattern matches the end of the variable's value, delete the shortest part that matches and return the rest.
${variable%%pattern}	If the pattern matches the end of the variable's value, delete the longest part that matches and return the rest.
${variable/pattern/string}${variable//pattern/string}	The longest match to pattern in variable is replaced by string. In the first form, only the first match is replaced. In the second form, all matches are replaced. If the pattern begins with a #, it must match at the start of the variable. If it begins with a %, it must match with the end of the variable. If string is null, the matches are deleted. If variable is @ or *, the operation is applied to each positional parameter in turn and the expansion is the resultant list.[6]

Table 4-1. Substitution operators

Operator	Substitution
${varname:-word}	If varname exists and isn't null, return its value; otherwise return word. Purpose: Returning a default value if the variable is undefined. Example: ${count:-0} evaluates to 0 if count is undefined.
${varname:=word}	If varname exists and isn't null, return its value; otherwise set it to word and then return its value. Positional and special parameters cannot be assigned this way. Purpose: Setting a variable to a default value if it is undefined. Example: ${count:=0} sets count to 0 if it is undefined.
${varname:?message}	If varname exists and isn't null, return its value; otherwise print varname: followed by message, and abort the current command or script (non-interactive shells only). Omitting message produces the default message parameter null or not set. Purpose: Catching errors that result from variables being undefined. Example: {count:?"undefined!"} prints "count: undefined!" and exits if count is undefined.
${varname:+word}	If varname exists and isn't null, return word; otherwise return null. Purpose: Testing for the existence of a variable. Example: ${count:+1} returns 1 (which could mean "true") if count is defined.
${varname:offset:length}	Performs substring expansion.[5] It returns the substring of $varname starting at offset and up to length characters. The first character in $varname is position 0. If length is omitted, the substring starts at offset and continues to the end of $varname. If offset is less than 0 then the position is taken from the end of $varname. If varname is @, the length is the number of positional parameters starting at parameter offset. Purpose: Returning parts of a string (substrings or slices). Example: If count is set to frogfootman, ${count:4} returns footman. ${count:4:4} returns foot.

Some examples of these include:

• *(alice|hatter|hare) would match zero or more occurrences of alice, hatter, and hare. So it would match the null string, alice, alicehatter, etc.

• +(alice|hatter|hare) would do the same except not match the null string.

• ?(alice|hatter|hare) would only match the null string, alice, hatter, or hare.

• @(alice|hatter|hare) would only match alice, hatter, or hare.

• !(alice|hatter|hare) matches everything except alice, hatter, and hare.

The values provided can contain shell wildcards too. So, for example, +([0-9]) matches a number of one or more digits. The patterns can also be nested, so you could remove all files except those beginning with vt followed by a number by doing rm !(vt+([0-9])).

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