Diff for "BashFAQ/067"

Differences between revisions 1 and 12 (spanning 11 versions)

How can I trim leading/trailing white space from one of my variables?

There are a few ways to do this:

   #POSIX, but fails if the variable contains newlines
   read -r var << EOF
   $var
   EOF

The easiest and cleanest way is with a bash herestring:

   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.

There's also a solution using extglob which shows how you can use it in parameter expansion:

   # Bash
   shopt -s extglob
   x=${x##+([[:space:]])} x=${x%%+([[:space:]])}
   shopt -u extglob

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

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

There are many, many other ways to do this, using sed for instance:

   # POSIX, suppress the trailing and leading whitespace on every lines
   x=$(echo "$x" | sed -e 's/^[[:space:]]*//' -e 's/[[:space:]]*$//')

-  ⇤ ← Revision 1 as of 2007-05-03 00:01:56 → 
  Size: 1281
  Editor: redondos
  Comment:
+   ← Revision 12 as of 2009-05-10 03:42:31 → ⇥
  Size: 1130
  Editor: localhost
  Comment: igli: *sigh* BASH handles strings fine. m00 gc ;-)
-Deletions are marked like this.
+Additions are marked like this.
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-[[Anchor(faq67)]]
+<<Anchor(faq67)>>
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-There are a few ways to do this -- none of them elegant.

First, the most portable way would be to use sed:
+There are a few ways to do this:
-Line 8:
+Line 6:
-   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
   $var
   EOF
-Line 15:
+Line 12:
-One can achieve the goal using builtins, although at the moment I'm not sure which shells the following syntax supports:
+The easiest and cleanest way is with a bash herestring:
{{{
   read  -rd '' x <<< "$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 18:
+There's also a solution using [[glob|extglob]] which shows how you can use it in parameter expansion:
-Line 18:
+Line 20:
-   # 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
-Line 28:
+Line 22:
-   x=${x##*([$' \t\n'])}; x=${x%%*([$' \t\n'])}
+   x=${x##+([[:space:]])} x=${x%%+([[:space:]])}
-Line 32:
+Line 26:
-There are many, many other ways to do this.  These are not necessarily the most efficient, but they're known to work.
+This also works in KornShell, without needing the explicit `extglob` setting:

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

There are many, many other ways to do this, using sed for instance:
{{{
   # POSIX, suppress the trailing and leading whitespace on every lines
   x=$(echo "$x" | sed -e 's/^[[:space:]]*//' -e 's/[[:space:]]*$//')
}}}