Regex sed issue

Question

My sed expression looks as belos:

sed -i "s/-D CONSOLELOG /-D CONSOLELOG  -fPIC /g" makefile.init

makefile.init

 CFLAGS = -std=c99 -rdynamic -g -Wall -Wno-write-strings -D CONSOLELOG

Output after 1st Run( As expected)

CFLAGS = -std=c99 -rdynamic -g -Wall -Wno-write-strings -D CONSOLELOG  -fPIC

2nd Run (Notice the extra fPIC at the end)

CFLAGS = -std=c99 -rdynamic -g -Wall -Wno-write-strings -D CONSOLELOG  -fPIC -fPIC

I need to modify my sed expression to get output as in (1) irrespective of the number of times it is executed

Or with `awk`, you could do `awk '$NF=="CONSOLELOG"{$(NF+1)="-fPIC"; NF++}1'` — Inian, May 21 '18 at 11:53
A simple alternative... you add a second `sed` in your script that collapses `-fPIC -fPIC` into `-fPIC`. — jww, May 22 '18 at 01:01

score 3 · Accepted Answer · answered May 21 '18 at 11:53

3

This might work for you (GNU sed):

sed -ri 's/-D CONSOLELOG (-fPIC )?$/&-fPIC /' file

This would insert at most 2 -fPIC options following a -D CONSOLELOG option.

answered May 21 '18 at 11:53

potong

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score 2 · Answer 2 · answered May 21 '18 at 12:35

Sample changed for illustration purposes

$ cat ip.txt 
42 foo baz 
ijk baz xyz
$ sed -i 's/baz $/&123/' ip.txt
$ cat ip.txt
42 foo baz 123
ijk baz xyz

$ # further runs won't change input
$ sed -i 's/baz $/&123/' ip.txt
$ cat ip.txt
42 foo baz 123
ijk baz xyz

$ is a meta character to ensure matching only at end of line
so, matches elsewhere in the line won't be changed and hence applying the command again won't result in duplication
& in replacement section is backreference to entire matched string in search section
since there can only be one match at end of line, g modifier is not needed

To replace anywhere in the line(assuming only single match per line)

$ cat ip.txt 
42 foo baz 
ijk baz xyz
$ sed -i '/baz 123/! s/baz /&123/' ip.txt
$ cat ip.txt
42 foo baz 123
ijk baz 123xyz

$ # further runs won't change input
$ sed -i '/baz 123/! s/baz /&123/' ip.txt
$ cat ip.txt
42 foo baz 123
ijk baz 123xyz

sed commands can be qualified with addressing
here, /baz 123/! means lines not matching baz 123

Further reading: Difference between single and double quotes in Bash

Regex sed issue

2 Answers2