Sed command to replace a variable's value in a config file with another variable's value in current script?

Question

My current script looks like this:

replacement="Hello"

sed -i 's/\(^TEXT_BLOCK=\).*/\1replacement/' path

I want to be able to replace the variable TEXT_BLOCK located in my config file with the contents of the variable replacement. I have tried \1$replacement and \1#replacement# but they do not give me my desired outcome.

Hardcoding it like this works for me but I do not want to hardcode. I want to be able to assign TEXT_BLOCK to a variable's value:

sed -i 's/\(^TEXT_BLOCK=\).*/\1Hello/' path

You need something like sed -i "s/^\$TEXT_BLOCK=\$.*/\\1$replacement/" path, double quotes and $var. — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Jul 23, 2021 at 16:05
@WiktorStribiżew unfortunately, this won't work for any value of $replacement; try e.g. with replacement="/"; so I'd suggest using a Perl one-liner or so, instead of sed here. — ErikMD
– ErikMD, Commented Jul 23, 2021 at 18:03

Walter A · Accepted Answer · 2021-07-26 22:15:28Z

1

For a simple replacement sed -i "s/^\$TEXT_BLOCK=\$.*/\\1$replacement/" path will work.
When replacement has a special character, you would like to use a different delimiter in the sed command.

replacement="with a /"
delim=$'\001'; sed 's'${delim}'\(^TEXT_BLOCK=\).*'${delim}'\1'"$replacement"${delim} path

However, this will still fail when the replacement has a &. This character will also cause problems in the "solution"

replacement="Hello"
t='TEXT_BLOCK'
awk -v r="${replacement}" -v key=${t} '{sub("^" key "=.*",key "=" r );print}'

When you want to catch such a replacement value too, you can use

replacement="Hello /& <- strangers"
t='TEXT_BLOCK'
awk -v r="${replacement//&/\\\\&}" -v key=${t} '{sub("^" key "=.*",key "=" r );print}'

edited Jul 26, 2021 at 22:15

answered Jul 24, 2021 at 16:12

Walter A

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ErikMD · Accepted Answer · 2021-07-24 15:24:22Z

0

Note that the sed one-liner suggested in the comments (sed -i "s/^\$TEXT_BLOCK=\$.*/\\1$replacement/" path) won't work for some values of $replacement.

E.g., if the variable contains a /, you'd get:

replacement="Hello/"
echo 'TEXT_BLOCK=init' | sed -e "s/^\\(TEXT_BLOCK=\\).*/\\1$replacement/"
# → sed: -e expression #1, char 30: unknown option to `s'

Albeit the OP's question is tagged with sed, I'd suggest to write some perl -wpe one-liner instead, to have a more robust script, and given perl is a fairly standard dependency, it should probably be available on the OP's target machine.

To write one such script, you could just rely on the Perl operators s/// and BEGIN{ }.

Proof of concept:

#!/usr/bin/env bash
subst() {
  re="$1"
  str="$2"
  perl -wpe 'BEGIN {$re=shift @ARGV; $str=shift @ARGV;}; s/$re/$str/g;' "$re" "$str"
}

# Usage example
echo $'TEXT_BLOCK="init"\nEnd.' | subst '^TEXT_BLOCK=.*$' 'TEXT_BLOCK="3$/"'

# will print:
#   TEXT_BLOCK="3$/"
#   End.

Note that this can easily be adapted to the sed -i use case:

#!/usr/bin/env bash
subst_i() {
  fil="$1"
  re="$2"
  str="$3"
  perl -i -wpe 'BEGIN {$re=shift @ARGV; $str=shift @ARGV;}; s/$re/$str/g;' "$re" "$str" "$fil"
}

# Usage example
echo $'TEXT_BLOCK="init"\nEnd.' > data.txt
subst_i data.txt '^TEXT_BLOCK=.*$' 'TEXT_BLOCK="3$/"'
cat data.txt

edited Jul 24, 2021 at 15:24

answered Jul 23, 2021 at 18:13

ErikMD

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1 Comment

ErikMD Over a year ago

BTW, regarding the issue mentioned in the recent awk answer, note that this perl answer is also robust w.r.t. strings containing the & character.

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