Shell – What Does `[$’\r\n’]` Mean?

escape-charactersshell

I regularly use the expression line=${line//[$'\r\n']}. But what does [$'\r\n'] mean?

I know it removes the '\r\n' characters but how does it do this? Does it remove the instances of both characters only, or does it also find matches of just one character?

I do not understand the usage of this syntax.

If you can, please, give me a link to the manual. I cannot find the answer on this question.

Best Answer

From the Bash manual:

${parameter/pattern/string}
The pattern is expanded to produce a pattern just as in filename expansion. Parameter is expanded and the longest match of pattern against its value is replaced with string. The match is performed according to the rules described below (see Pattern Matching). If pattern begins with /, all matches of pattern are replaced with string. ... If string is null, matches of pattern are deleted and the / following pattern may be omitted.

You have ${line//[$'\r\n']}, where:

the parameter is line,
the pattern is /[$'\r\n'] (note: begins with /, so all matches of pattern are replaced), and
the string is null, so the / after pattern is omitted, and matches are deleted.

Following the rules for Pattern Matching:

[…]
Matches any one of the enclosed characters.

$'...' tells bash to interpret certain escape sequences (here, \r for carriage return and \n for line feed) and replace them with actual characters represented by the escape sequences.

So this substitution matches all instances of either carriage return (CR, \r) or line feed (LF, \n), and deletes them.

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Bash – What does : ${param:=value} mean

Let's break this down into pieces.

This code runs the command : with some arguments. The command : does nothing and ignores its arguments. Therefore the whole command line does nothing, except whatever side effects happen in the arguments.

The syntax ${parameter_name:=value} exists in all non-antique Bourne-style shells, including ash, bash, ksh and zsh. It sets the parameter to a default if necessary. It is equivalent to

if [ -z "$parameter_name" ]; then parameter_name=value; fi
… ${parameter_name}

In other words, if parameter_name is not set or is set to an empty value, then set it to the indicated value; and then run the command, using the new parameter value. There is a variant, ${parameter_name=value}, which leaves the parameter empty if it was empty, only using the indicated value if the parameter was unset.

You'll find this syntax documented under “parameter expansion” in the POSIX spec, and the dash, bash, ksh and zsh manuals.

There are variations on this syntax, in particular ${parameter_name:-value} which let you use a default value for this expansion only, without assigning to the parameter.

In summary, : ${parameter_name:=value} is a concise way of writing

if [ -z "$parameter_name" ]; then parameter_name=value; fi

Bash – What does the expression ${MYVAR:+-x} mean in bash

This is the same as

${MYVAR:+OTHERVAR}

with OTHERVAR being equal to -x. In other words, if MYVAR is unset or null, substitute null; otherwise, substitute -x.

References

Parameter Expansion

Best Answer

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Bash – What does : ${param:=value} mean

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