Diff for "BashFAQ/067"

Differences between revisions 1 and 19 (spanning 18 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.

For simple variables, you can trim whitespace (or other characters) using this trick:

   # POSIX
   junk="${var##*[! ]}"   # match prefixed spaces
   var="${var%"$junk"}"    # trim the match
   junk="${var%%[! ]*}"   # match affixed spaces
   var="${var#"$junk"}"    # trim the match

Bash can do the same thing, but without the need for a throw-away variable, by using extglob's more advanced pattern matching:

   # Bash
   shopt -s extglob
   var="${var##*( )}"   # trim the left
   var="${var%%*( )}"   # trim the right

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:]])}

This 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
   done

Or 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.

-  ⇤ ← Revision 1 as of 2007-05-03 00:01:56 → 
  Size: 1281
  Editor: redondos
  Comment:
+   ← Revision 19 as of 2014-07-25 13:53:25 → ⇥
  Size: 2832
  Editor: geirha
  Comment:
-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:
+For simple variables, you can trim whitespace (or other characters) using this trick:
-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:]]\+$//')
+   # POSIX
   junk="${var##*[! ]}"   # match prefixed spaces
   var="${var%"$junk"}"    # trim the match
   junk="${var%%[! ]*}"   # match affixed spaces
   var="${var#"$junk"}"    # trim the match
-Line 15:
+Line 14:
-One can achieve the goal using builtins, although at the moment I'm not sure which shells the following syntax supports:
+Bash can do the same thing, but without the need for a throw-away variable, by using [[glob|extglob]]'s more advanced pattern matching:
-Line 18:
+Line 16:
-   # 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
   var="${var##*( )}"   # trim the left
   var="${var%%*( )}"   # trim the right
-Line 24:
+Line 22:
-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:
+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 27:
+Line 24:
-   shopt -s extglob
   x=${x##*([$' \t\n'])}; x=${x%%*([$' \t\n'])}
   shopt -u extglob
+   # POSIX, but fails if the variable contains newlines
   read -r var << EOF
   $var
   EOF
-Line 32:
+Line 30:
-There are many, many other ways to do this.  These are not necessarily the most efficient, but they're known to work.
+Bash can do something similar with a "here string":
{{{
   # Bash
   read  -rd '' x <<< "$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 [[glob|extglob]] together with  [[BashFAQ/073|parameter expansion]]:
{{{
   # Bash
   shopt -s extglob
   x=${x##+([[:space:]])} x=${x%%+([[:space:]])}
}}}

This 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 [[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.