How can I use variables in the LHS and RHS of a sed substitution?

Question

I want to do:

cat update_via_sed.sh | sed 's/old_name/new_name/' > new_update_via_sed.sh 

in my program.

But I want to use variables, e.g.

old_run='old_name_952' new_run='old_name_953' 

I have tried using them but the substitution doesn't happen (no error). I have tried:

cat update_via_sed.sh | sed 's/old_run/new_run/' cat update_via_sed.sh | sed 's/$old_run/$new_run/' cat update_via_sed.sh | sed 's/${old_run}/${new_run}/' 

Answer 1

You could do:

sed "s/$old_run/$new_run/" < infile > outfile 

But beware that $old_run would be taken as a regular expression and so any special characters that the variable contains, such as / or . would have to be escaped. Similarly, in $new_run, the & and \ characters would need to be treated specially and you would have to escape the / and newline characters in it.

If the content of $old_run or $new_run is not under your control, then it's critical to perform that escaping, or otherwise that code amounts to a code injection vulnerability.

Answer 2

This worked:

cat update_via_sed.sh | sed 's/'"$old_run"'/'"$new_run"'/' 

As I want to 'reuse' the same file I actually use this for anyone wishing a similar approach:

cat update_via_sed.sh | sed 's/'"$old_run"'/'"$new_run"'/' > new_update; mv -f new_update update_via_sed.sh 

The above created a new file then deletes the current file than rename the new file to be the current file.

Answer 3

You can use variables in sed as long as it's in a double quote (not a single quote):

sed "s/$var/r_str/g" file_name >new_file 

If you have a forward slash (/) in the variable then use different separator like below

sed "s|$var|r_str|g" file_name >new_file 

Answer 4

'in-place' sed (usng the -i flag) was the answer. Thanks to peterph.

sed -i "s@$old@$new@" file 

Answer 5

man bash gives this about single quoting

Enclosing characters in single quotes preserves the literal value of each character within the quotes. A single quote may not occur between single quotes, even when preceded by a backslash.

Whatever you type on the command line, bash interprets it and then it sends the result to the program it is supposed to be sent to.In this case, if you use sed 's/$old_run/$new_run/', bash first sees the sed, it recognises it as an executable present in $PATH variable. The sed executable requires an input. Bash looks for the input and finds 's/$old_run/$new_run/'. Single quotes say bash not to interpret the content in them and pass them as they are. So, bash then passes them to sed. Sed gives an error because $ can occur only at the end of line.

Instead if we use double quotes, i.e., "s/$old_run/$new_run/", then bash sees this and interprets $old_run as a variable name and makes a substitution (this phase is called variable expansion). This is indeed what we required.

But, you have to be careful using double quotes because, they are interpreted first by bash and then given to sed. So, some symbols like ` must be escaped before using them.

Answer 6

$ echo $d1 | sed -i 's/zkserver1/'${d1}'/g' deva_file sed: -e expression #1, char 26: unterminated `s' command 

In $d1 I am storing values

I cannot replace the value from the variable, so I tried the following command it is working:

echo $d1 | sed -i 's/zkserver1/'"${d1}"'/g' deva_file 

missing double quotation mark in "${d1}", that was the error.

Answer 7

At risk of being flippant - have you considered perl.

Which - amongst other things - is quite good at doing 'sed style' operations, but also a more fully featured programming thingummy.

It looks like in your example, what you're actually trying to do is increment a number.

So how about:

#!/usr/bin/env perl use strict; use warnings 'all'; while ( <DATA> ) { s/old_name_(\d+)/"old_name_".($1+1)/e; print; } __DATA__ old_name_932 

or as a one liner - sed style:

perl -pe 's/old_name_(\d+)/"old_name_".($1+1)/e' 

Answer 8

break the string

bad => linux see a string $old_run :

sed 's/$old_run/$new_run/' 

good => linux see a variable $old_run:

sed 's/'$old_run'/'$new_run'/' 

Answer 9

You can also use Perl:

perl -pi -e ""s/$val1/$val2/g"" file