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| There are a few ways to do this -- none of them elegant. | There are a few ways to do this. Some involve special tricks that only work with whitespace. Others are more general, and can be used to strip leading zeroes, etc. |
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| First, the most portable way would be to use sed: |
Here's one that only works for whitespace. It relies on the fact that `read` strips all leading and trailing whitespace when `IFS` isn't set: |
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| x=$(echo "$x" | sed -e 's/^ *//' -e 's/ *$//') # Note: this only removes spaces. For tabs too: x=$(echo "$x" | sed -e $'s/^[ \t]*//' -e $'s/[ \t]*$//') # Or possibly, with some systems: x=$(echo "$x" | sed -e 's/^[[:space:]]*//' -e 's/[[:space:]]*$//') |
# POSIX, but fails if the variable contains newlines read -r var << EOF $var EOF |
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| One can achieve the goal using builtins, although at the moment I'm not sure which shells the following syntax supports: | Bash can do something similar with a "here string": {{{ # Bash read -rd '' x <<< "$x" }}} Using an empty string as a delimiter means the read consumes the whole string, as NUL is used. (Remember: BASH only does C-string variables.) This is entirely safe for any text, including newlines. |
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| Here's a solution using [[glob|extglob]] together with [[BashFAQ/073|parameter expansion]]: | |
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| # Remove leading whitespace: while [[ $x = [$' \t\n']* ]]; do x=${x#[$' \t\n']}; done # And now trailing: while [[ $x = *[$' \t\n'] ]]; do x=${x%[$' \t\n']}; done }}} Of course, the preceding example is pretty slow, because it removes one character at a time, in a loop (although it's good enough in practice for most purposes). If you want something a bit fancier, there's a bash-only solution using extglob: {{{ |
# Bash |
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| x=${x##+([$' \t\n'])}; x=${x%%+([$' \t\n'])} | x=${x##+([[:space:]])} x=${x%%+([[:space:]])} |
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| Rather than specify each type of space character yourself, you can use character classes. Two character classes that are useful for matching whitespace are space and blank. More info: ctype/wctype(3), re_format/regex(7), isspace(3). |
This also works in KornShell, without needing the explicit `extglob` setting: |
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| shopt -s extglob x=${x##+([[:space:]])}; x=${x%%+([[:space:]])} shopt -u extglob |
# ksh x=${x##+([[:space:]])} x=${x%%+([[:space:]])} |
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| There are many, many other ways to do this. These are not necessarily the most efficient, but they're known to work. | This solution isn't restricted to whitespace like the first few were. You can remove leading zeroes as well: {{{ # Bash shopt -s extglob x=${x##+(0)} }}} Another way to remove leading zeroes from a number in bash is to treat it as an integer, in a [[ArithmeticExpression|math context]]: {{{ # Bash x=$((10#$x)) # However, this fails if x contains anything other than digits. }}} If you need to remove leading zeroes in a POSIX shell, you can use a loop: {{{ # POSIX while true; do case "$var" in 0*) var=${var#0};; *) break;; esac done }}} Or this trick (covered in more detail in [[BashFAQ/100|FAQ #100]]): {{{ # POSIX zeroes=${var%%[!0]*} var=${var#$zeroes} }}} There are many, many other ways to do this, using sed for instance: {{{ # POSIX, suppress the trailing and leading whitespace on every line x=$(echo "$x" | sed -e 's/^[[:space:]]*//' -e 's/[[:space:]]*$//') }}} Solutions based on external programs like sed are better suited to trimming large files, rather than shell variables. |
How can I trim leading/trailing white space from one of my variables?
There are a few ways to do this. Some involve special tricks that only work with whitespace. Others are more general, and can be used to strip leading zeroes, etc.
Here's one that only works for whitespace. It relies on the fact that read strips all leading and trailing whitespace when IFS isn't set:
# POSIX, but fails if the variable contains newlines read -r var << EOF $var EOF
Bash can do something similar with a "here string":
# Bash read -rd '' x <<< "$x"
Using an empty string as a delimiter means the read consumes the whole string, as NUL is used. (Remember: BASH only does C-string variables.) This is entirely safe for any text, including newlines.
Here's a solution using extglob together with parameter expansion:
# Bash
shopt -s extglob
x=${x##+([[:space:]])} x=${x%%+([[:space:]])}
shopt -u extglobThis also works in KornShell, without needing the explicit extglob setting:
# ksh
x=${x##+([[:space:]])} x=${x%%+([[:space:]])}This solution isn't restricted to whitespace like the first few were. You can remove leading zeroes as well:
# Bash
shopt -s extglob
x=${x##+(0)}Another way to remove leading zeroes from a number in bash is to treat it as an integer, in a math context:
# Bash x=$((10#$x)) # However, this fails if x contains anything other than digits.
If you need to remove leading zeroes in a POSIX shell, you can use a loop:
# POSIX
while true; do
case "$var" in
0*) var=${var#0};;
*) break;;
esac
doneOr this trick (covered in more detail in FAQ #100):
# POSIX
zeroes=${var%%[!0]*}
var=${var#$zeroes}There are many, many other ways to do this, using sed for instance:
# POSIX, suppress the trailing and leading whitespace on every line x=$(echo "$x" | sed -e 's/^[[:space:]]*//' -e 's/[[:space:]]*$//')
Solutions based on external programs like sed are better suited to trimming large files, rather than shell variables.