Diff for "BashFAQ/067"

Differences between revisions 10 and 16 (spanning 6 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:]])}

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 10 as of 2008-07-17 15:56:25 → 
  Size: 1107
  Editor: wsip-68-15-32-50
  Comment:
+   ← Revision 16 as of 2011-06-21 15:20:23 → ⇥
  Size: 2264
  Editor: GreyCat
  Comment: don't bother turning off extglob afterward
-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:
+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:
+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:
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+Line 7:
-   #POSIX, but fails if the variable contains newlines
+   # POSIX, but fails if the variable contains newlines
-Line 8:
+Line 9:
-   $var 
   EOF
+   $var
   EOF
-Line 12:
+Line 13:
-One can also achieve in bash using a herestring
+Bash can do something similar with a "here string":
-Line 14:
+Line 15:
-   # Bash still fails if the variable contains a newline.
   read  -r x <<< "$x"
+   # Bash
   read  -rd '' x <<< "$x"
-Line 17:
+Line 18:
+Using an empty string as a delimiter means the read consumes the whole string, as NUL is used. (Remember: BASH only does C-string variables.)  This is entirely safe for any text, including newlines.
-Line 18:
+Line 20:
-(note: using IFS=$' \t' read -d "" -r x partially fix the "problem" of the newlines but adds a trailing \n)

There's also a solution using [:glob:extglob]:
+Here's a solution using [[glob|extglob]] together with  [[BashFAQ/073|parameter expansion]]:
-Line 26:
+Line 25:
-   shopt -u extglob
-Line 30:
+Line 28:
-Line 36:
+Line 33:
-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:
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+Line 35:
-   # POSIX, suppress the trailing and leading whitespace on every lines
+   # 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
-Line 41:
+Line 70:
-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.