Diff for "BashFAQ/067"

Differences between revisions 6 and 17 (spanning 11 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 6 as of 2008-07-04 09:01:07 → 
  Size: 883
  Editor: pgas
  Comment: posix solution with a here string
+   ← Revision 17 as of 2013-08-23 15:09:01 → ⇥
  Size: 2828
  Editor: ghoti
  Comment: add a clearer whitespace option in POSIX and Bash, at the top of the page.
-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
+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:
-Line 5:
+Line 7:
-   #POSIX
   read -r var << EOF
   $var 
   EOF
+   # POSIX
   junk="${var##*[! ]}"   # match prefixed spaces
   var="${var%%$junk}"    # trim the match
   junk="${var%%[! ]*}"   # match affixed spaces
   var="${var##$junk}"    # trim the match
-Line 11:
+Line 14:
-One can also achieve in bash (and ksh93) using a herestring
+Bash can do the same thing, but without the need for a throw-away variable, by using [[glob|extglob]]'s more advanced pattern matching:
-Line 15:
+Line 17:
-   # Remove leading whitespace:
   read -r x <<< "$x"
+   shopt -s extglob
   var="${var##*( )}"   # trim the left
   var="${var%%*( )}"   # trim the right
-Line 19:
+Line 22:
-There's also a solution using [:glob: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:
{{{
   # POSIX, but fails if the variable contains newlines
   read -r var << EOF
   $var
   EOF
}}}
-Line 21:
+Line 30:
+Bash can do something similar with a "here string":
{{{
   # Bash
   read  -rd '' x <<< "$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]]:
-Line 25:
+Line 42:
-   shopt -u extglob
-Line 29:
+Line 45:
-Line 35:
+Line 50:
-There are many, many other ways to do this, using sed for instance:
+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:
-Line 38:
+Line 67:
+   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
-Line 40:
+Line 87:
-These are not necessarily the best, but they're known to work.
+Solutions based on external programs like sed are better suited to trimming large files, rather than shell variables.