Diff for "BashFAQ/067"

Differences between revisions 4 and 14 (spanning 10 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 4 as of 2008-05-21 20:52:00 → 
  Size: 1195
  Editor: GreyCat
  Comment: clean up
+   ← 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:
-   # POSIX
   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 12:
+Line 13:
-One can also achieve the same goal using Bash builtins:
+Bash can do something similar with a "here string":
 Line 16:
-   # Remove leading whitespace:
   while [[ $x = [[:space:]]* ]]; do x=${x#[[:space:]]}; done
+   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 19:
+Line 20:
-   # And now trailing:
   while [[ $x = *[[:space:]] ]]; do x=${x%[[:space:]]}; done
}}}

Of course, the preceding example is not optimal, 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 solution using [:glob:extglob]:
+Here's a solution using [[glob|extglob]] together with  [[BashFAQ/073|parameter expansion]]:
-Line 31:
+Line 27:
+This solution isn't restricted to whitespace like the first few were.  You can remove leading zeroes as well:
{{{
   # Bash
   shopt -s extglob
   x=${x##+(0)}
}}}
-Line 33:
+Line 35:
-Line 39:
+Line 40:
-There are many, many other ways to do this.  These are not necessarily the best, but they're known to work.
+If you need to remove leading zeroes in a POSIX shell, you can use a loop:
{{{
   # POSIX
   while true; do
     case "$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.