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I am trying to change the values in a text file using sed in a Bash script with the line,

sed 's/draw($prev_number;n_)/draw($number;n_)/g' file.txt > tmp

This will be in a for loop. Why is it not working?

Peter Mortensen
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csta
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5 Answers5

93

Variables inside ' don't get substituted in Bash. To get string substitution (or interpolation, if you're familiar with Perl) you would need to change it to use double quotes " instead of the single quotes:

$ # Enclose the entire expression in double quotes
$ sed "s/draw($prev_number;n_)/draw($number;n_)/g" file.txt > tmp

$ # Or, concatenate strings with only variables inside double quotes
$ # This would restrict expansion to the relevant portion
$ # and prevent accidental expansion for !, backticks, etc.
$ sed 's/draw('"$prev_number"';n_)/draw('"$number"';n_)/g' file.txt > tmp

$ # A variable cannot contain arbitrary characters
$ # See link in the further reading section for details
$ a='foo
bar'
$ echo 'baz' | sed 's/baz/'"$a"'/g'
sed: -e expression #1, char 9: unterminated `s' command


Further Reading:

Peter Mortensen
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k.parnell
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    Maybe also mention that if the variable contains slashes, you need to use a different delimiter which doesn't occur in the string (or backslash-escape every occurrence of the delimiter). The slash is a common default for `s` but you can use any punctuation character; `sed "s%draw($prev_number;n_)%draw($number;n_)%g" file.txt` or `s#from#to#g` or `s_from_to_g` etc. – tripleee Jun 25 '15 at 07:48
  • it doesn't work if the variable contains new-lines ("\n") – dentex Apr 02 '18 at 10:06
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    but it did once escaped (for my use cases) also slashes: `changelogEscaped=$(sed -e 's%\/%\\/%g; $!a\'$'\n''\\n' << – dentex Apr 02 '18 at 10:19
14

Variables within single quotes are not expanded, but within double quotes they are. Use double quotes in this case.

sed "s/draw($prev_number;n_)/draw($number;n_)/g" file.txt > tmp

You could also make it work with eval, but don’t do that!!

Peter Mortensen
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Paul Creasey
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  • Thanks that worked. Would you mind explaining what you mean by expanded? My intuition tells me that it should be something like, variable substituted. Also why not use eval? – csta Oct 07 '11 at 12:52
5

This may help:

sed "s/draw($prev_number;n_)/draw($number;n_)/g"
Kent
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1

You can use variables like below. Like here, I wanted to replace hostname i.e., a system variable in the file. I am looking for string look.me and replacing that whole line with look.me=<system_name>

sed -i "s/.*look.me.*/look.me=`hostname`/"

You can also store your system value in another variable and can use that variable for substitution.

host_var=`hostname`

sed -i "s/.*look.me.*/look.me=$host_var/"

Input file:

look.me=demonic

Output of file (assuming my system name is prod-cfm-frontend-1-usa-central-1):

look.me=prod-cfm-frontend-1-usa-central-1
Peter Mortensen
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daemonsl
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1

I needed to input github tags from my release within github actions. So that on release it will automatically package up and push code to artifactory.

Here is how I did it. :)

  - name: Invoke build
    run: |
      # Gets the Tag number from the release
      TAGNUMBER=$(echo $GITHUB_REF | cut -d / -f 3)
      
      # Setups a string to be used by sed
      FINDANDREPLACE='s/${GITHUBACTIONSTAG}/'$(echo $TAGNUMBER)/
      
      # Updates the setup.cfg file within version number
      sed -i $FINDANDREPLACE setup.cfg
      
      # Installs prerequisites and pushes 
      pip install -r requirements-dev.txt
      invoke build

Retrospectively I wish I did this in python with tests. However it was fun todo some bash.

James Green
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