Diff for "BashFAQ/067"

Differences between revisions 2 and 14 (spanning 12 versions)

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 extglob

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)}

This also works in KornShell, without needing the explicit extglob setting:

   # ksh
   x=${x##+([[:space:]])} x=${x%%+([[:space:]])}

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:

   # 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 well suited to trimming large files, rather than shell variables.

-  ⇤ ← Revision 2 as of 2007-06-26 00:17:35 → 
  Size: 1625
  Editor: cpe-74-65-28-251
  Comment: ${x##*([$' \t\n'])} is wrong -- you don't want to match zero or more of those characters. You want +, for one or more.
+   ← Revision 14 as of 2009-11-04 15:22:51 → ⇥
  Size: 1986
  Editor: GreyCat
  Comment: add non-loop POSIX trick
-Deletions are marked like this.
+Additions are marked like this.
 Line 1:
-[[Anchor(faq67)]]
+<<Anchor(faq67)>>
 Line 3:
-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.
 Line 5:
-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:
-Line 8:
+Line 7:
-   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
   $var
   EOF
-Line 15:
+Line 13:
-One can achieve the goal using builtins, although at the moment I'm not sure which shells the following syntax supports:
+Bash can do something similar with a "here string":
{{{
   # Bash
   read  -rd '' x <<< "$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.
-Line 17:
+Line 20:
+Here's a solution using [[glob|extglob]] together with  [[BashFAQ/073|parameter expansion]]:
-Line 18:
+Line 22:
-   # 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
+   # Bash
   shopt -s extglob
   x=${x##+([[:space:]])} x=${x%%+([[:space:]])}
   shopt -u extglob
}}}
This solution isn't restricted to whitespace like the first few were.  You can remove leading zeroes as well:
{{{
   # Bash
   shopt -s extglob
   x=${x##+(0)}
-Line 24:
+Line 34:
-Of course, the preceding example is pretty slow, because it removes one character at a time, in a loop (although it's good enough in practice for most purposes).  If you want something a bit fancier, there's a bash-only solution using extglob:
+This also works in KornShell, without needing the explicit `extglob` setting:
-Line 27:
+Line 36:
-   shopt -s extglob
   x=${x##+([$' \t\n'])}; x=${x%%+([$' \t\n'])}
   shopt -u extglob
+   # ksh
   x=${x##+([[:space:]])} x=${x%%+([[:space:]])}
-Line 32:
+Line 40:
-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).
+If you need to remove leading zeroes in a POSIX shell, you can use a loop:
-Line 37:
+Line 42:
-   shopt -s extglob
   x=${x##+([[:space:]])}; x=${x%%+([[:space:]])}
   shopt -u extglob
+   # POSIX
   while true; do
     case "$var" in
       0*) var=${var#0};;
       *)  break;;
     esac
   done
-Line 42:
+Line 51:
-There are many, many other ways to do this.  These are not necessarily the most efficient, but they're known to work.
+Or this trick:
{{{
   # 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 well suited to trimming large files, rather than shell variables.