Swapping Two Strings using Sed

Question

I have a file in which I am trying to change two strings or trying to swapping with each other..

cat filename| grep m07

filesystem special="/abc/ca97" raw="/abc/ca97" directory="/mount/m07" 
filesystem special="/abc/d1107" raw="/abc/d1107" directory="/m07"

so I am trying to swap /mount/m07 with /m07 so that required output should be after operation:

filesystem special="/abc/ca97" raw="/abc/ca97" directory="/m07"
filesystem special="/abc/d1107" raw="/abc/d1107" directory="/mount/m07"

I tried it with using sed..

sed -e 's/"\/mount\/m07/"\m07/g' -e 's/"\/m07/"\/mount/\/m07' file_name

and

sed -e 's/"\/m07/"\/mount/\/m07' -e 's/"\/mount\/m07/"\m07/g' file_name

But in both the cases its replacing both the strings so either I get /mount/m07 or /m07 in both of the lines...

could you please suggest the path to reach the desired output...

RavinderSingh13 · Answer 1 · 2018-01-19T17:32:01.163

4

Simple sed will do the task for you.

sed '/m07/s#/mount/m07#/m07#;t;/m07/s#/m07#/mount/m07#'    Input_file

For Input_file you mentioned or command's output, following will be the output:

your_command | sed '/m07/s#/mount/m07#/m07#;n;/m07/s#/m07#/mount/m07#' 
<filesystem special="/abc/ca97" raw="/abc/ca97" directory="/m07" >
<filesystem special="/abc/d1107" raw="/abc/d1107" directory="/m07">

EDIT: Adding explanation too now for above command, THANKS to Sundeep for letting me know that t option is better than n. Also following code is only for explanation purposes not for running.

sed '/m07/                    Searching for string m07 in a line, if it is present then only do substitutions in that line.
s#/mount/m07#/m07#;           Doing substitution here substitute /mount/m07 with m07 here
t;                            t is for(from man sed page) If  a s/// has done a successful substitution since the last input line was read and since the last t or T command, then branch to label if label is omitted, branch to end of script.
/m07/s#/m07#/mount/m07#       searching string m07 and then substitute string /m07 with /mount/m07
'    Input_file               Mentioning Input_file.

edited Jan 19 '18 at 17:32

answered Jan 19 '18 at 15:25

RavinderSingh13

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@User123, in `sed` we could create our own separators too eg--> `sed '/m07/s@/mount/m07@/m07@' Input_file`. – RavinderSingh13 Jan 19 '18 at 15:36
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looks like everybody missed the question - it says swap... so `/mount/m07` is changed to `/m07` and `/m07` is changed to `/mount/m07` – Sundeep Jan 19 '18 at 15:39
well, i tried the command you mentioned but it showing /m07 in both lines, whereas I am trying to swap them to get this output.. filesystem special="/abc/ca97" raw="/abc/ca97" directory="/m07" filesystem special="/abc/d1107" raw="/abc/d1107" directory="/mount/m07" – Sandy Jan 19 '18 at 15:40
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@RavinderSingh13 using `n` will work for given example, but not a generic solution.. just add a line in between given sample and you'll see – Sundeep Jan 19 '18 at 15:47
@Sundeep, thanks a TON sir for letting know here, changed it in code. – RavinderSingh13 Jan 19 '18 at 17:32
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@RavinderSingh13: thanks Ravinder for the explanation.. :) – Sandy Jan 22 '18 at 10:01

Sundeep · Answer 2 · 2018-01-19T16:13:58.280

2

question is tagged linux, so am assuming GNU sed is available (not sure if below solution is POSIX). also assuming that both the strings are not present in same line

$ sed 's#/mount/m07#/m07#; t; s#/m07#/mount/m07#' ip.txt
filesystem special="/abc/ca97" raw="/abc/ca97" directory="/m07" 
filesystem special="/abc/d1107" raw="/abc/d1107" directory="/mount/m07"

t command will branch to specified label on successful substitution
Without label, t will skip rest of commands and start next cycle
so, if s#/mount/m07#/m07# succeeds, t command will tell sed to skip rest of commands and read next line for processing
but if the first substitution doesn't succeed, then the second substitution would be attempted

for non-GNU versions - thanks @anubhava

sed -e 's#/mount/m07#/m07#;t' -e 's#/m07#/mount/m07#'
#or
sed -e 's#/mount/m07#/m07#' -e 't' -e 's#/m07#/mount/m07#'

Further Reading

edited Jan 19 '18 at 16:13

answered Jan 19 '18 at 15:43

Sundeep

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++ Equivalent POSIX `sed` woule be: `sed -e 's#/mount/m07#/m07#;t' -e 's#/m07#/mount/m07#' file` – anubhava Jan 19 '18 at 15:49
@anubhava Why the first `s` and `t` can be together but the second `s` needs to be separated? – PesaThe Jan 19 '18 at 16:00
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Because POSIX sed attempts to find anything as `label` that comes after `t`. Even a `;` after `t` fails POSIX sed. – anubhava Jan 19 '18 at 16:02
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@PesaThe see https://www.gnu.org/software/sed/manual/sed.html#Multiple-commands-syntax – Sundeep Jan 19 '18 at 16:02
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@anubhava Sundeep: I see! Thanks :) – PesaThe Jan 19 '18 at 16:05

score 0 · Answer 3 · answered Jan 19 '18 at 16:03

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$ cat sedswap.txt 
/mount/m07
/m07

$ sed -e 's/\/mount\/m07/\/rm07/g' -e 's/\/m07/\/mount\/m07/g' -e 's/\/rm07/\/m07/g' sedswap.txt
/m07
/mount/m07

Basically using a temp variable approach

answered Jan 19 '18 at 16:03

SouravP

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Or like others previously have mentioned use a custom delimiter in sed `$ sed -e 's#/mount/m07#/rm07#g' -e 's#/m07#/mount/m07#g' -e 's#/rm07#/m07#g' sedswap.txt` – SouravP Jan 19 '18 at 16:23

Swapping Two Strings using Sed

3 Answers3