Bash: Preserving Whitespace Using set and eval
November 5th, 2008 by Mitch Frazier in
If you don't care much about whitespace bash is great: it normally turns multiple whitespace characters into one and it breaks things into words based on white space. If on the other hand you'd like to preserve whitespace bash can be a bit difficult at times. A trick which often helps is using a combination of bash's eval and set commands.
Let's say that you're building a list of items where each item may contain significant spaces, say something like:
#!/bin/bash
items=
for i in "$@"
do
items="$items \"$i\""
done
for i in $items
do
echo $i
done
But when you run this and try to use the items from the saved list you don't quite get what you expected:
$ sh t1.sh "ab cd" "ef gh" "ab cd" "ef gh"
One solution is to do the following:
#!/bin/bash
items=
for i in "$@"
do
items="$items \"$i\""
done
eval set -- $items
for i in "$@"
do
echo $i
done
Which produces the desired result:
$ sh t2.sh "ab cd" "ef gh" ab cd ef gh
The important line is:
eval set -- $items
The set command takes any arguments after the options (here "--" signals the end of the options) and assigns them to the positional parameters ($0..$n). The eval command executes its arguments as a bash command.
If you do this without the eval command you'll get the same result as the first example. By passing the set command to eval bash will honor the embedded quotes in the string rather than assume they are part of the word.
If you run this script you can see a bit more of what bash is doing:
#!/bin/bash
items=
for i in "$@"
do
items="$items \"$i\""
done
set -x
set -- $items
set +x
echo '===='
set -x
eval set -- $items
set +x
This produces:
$ sh t3.sh "ab cd" "ef gh" + set -- '"ab' 'cd"' '"ef' 'gh"' + set +x ==== + eval set -- '"ab' 'cd"' '"ef' 'gh"' ++ set -- 'ab cd' 'ef gh' + set +x__________________________
Mitch Frazier is an Associate Editor for Linux Journal and the Web Editor for linuxjournal.com.
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What about non-quoted?
On November 16th, 2008 Anonymous (not verified) says:
The IFS seems to fix the multiple space problem:
------------------------
#!/bin/bash
IFS=
while read line
do
echo $line
done
------------------------
This also does not need quoted input.
On the example of reading from a file
On November 16th, 2008 Anonymous (not verified) says:
Suppose the user types
"ab"
"a b"
"a b"
"a b"
for stdin. We get:
ab
a b
a b
a b
I.e., multiple spaces are reduced to one space.
Embedded Quotes
On November 6th, 2008 Anonymous (not verified) says:
The example given has problems with inputs that contain quotes
$ t1.sh "foo says, \"bar\""gives
foo says, barTo fix this, the inherent quote in each element needs to be replaced. The resulting script:
#!/bin/bash
items=
for i in "$@"
do
items="$items \"$(echo $i | sed 's/"/\\"/g')\""
done
eval set -- $items
for i in "$@"
do
echo $i
done
gives the desired result:
foo says, "bar"Simple way
On November 6th, 2008 Anonymous (not verified) says:
A simple way would be to use an array :
items=("$@") for i in "${items[@]}"; do echo $i donexavier
For this example
On November 6th, 2008 Mitch Frazier says:
Using an array is certainly an option in the example I used, but suppose the data with significant spaces is being read from a file or from the terminal such as in this example:
If the user types "ab cd" "ef gh" and we'd like to display ab cd and ef gh (without quotes) then you need eval. Another alternative that does use arrays is the following:
Mitch Frazier is an Associate Editor for Linux Journal and the Web Editor for linuxjournal.com.
Thanks for clarifying that
On November 10th, 2008 lening geld (not verified) says:
Thanks for clarifying that part!
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