Bash – Word-splitting when parameter is used within command substitution

bashcommand-substitutionquotingshell

What needs to be done to ensure that a parameter containing embedded spaces is handled correctly when used within a command substitution? Perhaps an example will illustrate

$ file="/home/1_cr/xy z"
$ basename $file
xy

$ #need to pass basename a quoted $file to prevent the z in 'xy z' from being eaten
$ basename "$file"
xy z

$ #Now using $file inside a command substitution without quoting.....
$ #....the command substitution causes 'xy z' to split
$ set -- $(basename "$file")
$ printf "%s\n" "$@"
xy
z

$ #But quoting the command substitution does not prevent 'xy z'...... 
$ #....from being split before being passed to basename
$ set -- "$(basename $file)"
$ printf "%s\n" "$@"
xy

What do i need to do so that

$ set -- $(basename $file)
$ printf "%s\n" "$@"

yields

xy z

Best Answer

You need to put double quotes around all variable and command substitutions.

set -- "$(basename -- "$file")"

If you allow the value of the variable to be split into words and these words to be treated as glob patterns, there's no going back. You can no longer tell how much whitespace was there or whether the file names were the result of a wildcard expansion. You can't turn the hamburger back into a cow, so you must make sure not to send the cow to the abattoir in the first place.

Related Solutions

Bash – Command Substitution Splitting on Newline but Not Space

Portably:

set -f              # turn off globbing
IFS='
'                   # split at newlines only
cmd $(cat <file)
unset IFS
set +f

Or using a subshell to make the IFS and option changes local:

( set -f; IFS='
'; exec cmd $(cat <file) )

The shell performs field splitting and filename generation on the result of a variable or command substitution that is not in double quotes. So you need to turn off filename generation with set -f, and configure field splitting with IFS to make only newlines separate fields.

There's not much to be gained with bash or ksh constructs. You can make IFS local to a function, but not set -f.

In bash or ksh93, you can store the fields in an array, if you need to pass them to multiple commands. You need to control expansion at the time you build the array. Then "${a[@]}" expands to the elements of the array, one per word.

set -f; IFS=$'\n'
a=($(cat <file))
set +f; unset IFS
cmd "${a[@]}"

Bash – Quoting Within $(command substitution)

In order from worst to best:

DIRNAME="$(dirname $FILE)" will not do what you want if $FILE contains whitespace (or whatever characters $IFS currently contains) or globbing characters \[?*.
DIRNAME=`dirname "$FILE"` is technically correct, but backticks are not recommended for command expansion because of the extra complexity when nesting them and the extra backslash processing that happens within them.
DIRNAME=$(dirname "$FILE") is correct, but only because this is an assignment to a scalar (not array) variable. If you use the command substitution in any other context, such as export DIRNAME=$(dirname "$FILE") or du $(dirname -- "$FILE"), the lack of quotes will cause trouble if the result of the expansion contain whitespace or globbing characters.
DIRNAME="$(dirname "$FILE")" (except for the missing --, see below) is the recommended way. You can replace DIRNAME= with a command and a space without changing anything else, and dirname receives the correct string.

To improve even further:

DIRNAME="$(dirname -- "$FILE")" works if $FILE starts with a dash.
DIRNAME="$(dirname -- "$FILE" && printf x)" && DIRNAME="${DIRNAME%?x}" || exit works even if $FILE's dirname ends with a newline, since $() chops off newlines at the end of output, both the one added by dirname and the ones that may be part of the actual data.

You can nest command expansions as much as you like. With $() you always create a new quoting context, so you can do things like this:

foo "$(bar "$(baz "$(ban "bla")")")"

You do not want to try that with backticks.

Best Answer

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Bash – Command Substitution Splitting on Newline but Not Space

Bash – Quoting Within $(command substitution)

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