sed + add string on the beginning of line but ignore if already exist

Question

here is example how to add string on the beginning of line when match the UUID number in fstab

sed -e "/UUID=953b1921-ac73-4b7b-abaf-ff983b0fce8a/ s/^/###FAULTY_DISK###/"  -i /etc/fstab

and we can verify with

more /etc/fstab
###FAULTY_DISK###UUID=953b1921-ac73-4b7b-abaf-ff983b0fce8a /data/sdc ext4 defaults,noatime 0 0

but on the second running we get

sed -e "/UUID=953b1921-ac73-4b7b-abaf-ff983b0fce8a/ s/^/###FAULTY_DISK###/"  -i /etc/fstab
more /etc/fstab
###FAULTY_DISK######FAULTY_DISK###UUID=953b1921-ac73-4b7b-abaf-ff983b0fce8a /data/sdc ext4 defaults,noatime 0 0

how to tell sed to ignore the adding of ###FAULTY_DISK### , in case it already set in file fstab

Does this answer your question? comment the specific line in fstab file that contained the relevant UUID number The accepted answer does what you want, and does not add the comment character (which could be a longer string if you wish) more than once upon re-running. — Kusalananda, Feb 02 '22 at 18:00

score 8 · Accepted Answer · edited Feb 03 '22 at 13:06

8

Append only if it was starting with that UUID:

sed -e 's/^UUID=953b1921-ac73-4b7b-abaf-ff983b0fce8a/###FAULTY_DISK###&/' /etc/fstab

or, in case you wanted to ignore leading whitespace too if any:

sed -e 's/^[[:blank:]]*UUID=953b1921-ac73-4b7b-abaf-ff983b0fce8a/###FAULTY_DISK###&/' /etc/fstab

edited Feb 03 '22 at 13:06

terdon

242,166

answered Feb 02 '22 at 17:12

αғsнιη

41,407

terdon · Answer 2 · 2022-02-03T13:06:07.957

Another approach is to just skip lines with ###FAULTY_DISK###. Consider this example file that has one line with the faulty disk comment and another without:

$ cat fstab 
UUID=953b1921-ac73-4b7b-abaf-ff983b0fce8a /data/sdc ext4 defaults,noatime 0 0
###FAULTY_DISK###UUID=953b1921-ac73-4b7b-abaf-ff983b0fce8a /data/sdc ext4 defaults,noatime 0 0

You can use the n command in sed to move to the next line. So tell sed to move to the next line if this one matches the faulty message:

$ sed  "/###FAULTY_DISK###/n; s/UUID=953b1921-ac73-4b7b-abaf-ff983b0fce8a/###FAULTY_DISK###&/" fstab 
###FAULTY_DISK###UUID=953b1921-ac73-4b7b-abaf-ff983b0fce8a /data/sdc ext4 defaults,noatime 0 0
###FAULTY_DISK###UUID=953b1921-ac73-4b7b-abaf-ff983b0fce8a /data/sdc ext4 defaults,noatime 0 0

As you can see, the message was only added to the line that did not have it.

You'd want to use b instead of n here (and use two -expressions). With n here, if both line 1 and line 2 contain FAULTY_DISK and line 2 contains the UUID, you'll add an extra FAULTY_DISK — Stéphane Chazelas, Feb 07 '22 at 08:14

score 1 · Answer 3 · answered Feb 02 '22 at 17:42

1

If I understand correctly:

Given

##FAULTY_DISK###UUID=953b1921-ac73-4b7b-abaf-ff983b0fce8a
UUID=953b1921-ac73-4b7b-abaf-ff983b0fce8a

Then

$ sed '/^###FAULTY_DISK###UUID=953b1921-ac73-4b7b-abaf-ff983b0fce8a/!s/^UUID=953b1921-ac73-4b7b-abaf-ff983b0fce8a/###FAULTY_DISK###&/' /etc/fstab 
###FAULTY_DISK###UUID=953b1921-ac73-4b7b-abaf-ff983b0fce8a
###FAULTY_DISK###UUID=953b1921-ac73-4b7b-abaf-ff983b0fce8a

answered Feb 02 '22 at 17:42

schrodingerscatcuriosity

12,396

This answer would be improved by explaining what ! does and why it fixes the issue. – David Conrad Feb 03 '22 at 15:47

score 0 · Answer 4 · answered Feb 07 '22 at 05:02

Using Raku (formerly known as Perl_6)

raku -pe 's/ ^ ("UUID=86d58af9-801b-4c25-b59d-80b52b4acc61") /\#\#\#FAULTY_DISK\#\#\#$0/;' fstab_test.txt

OR

raku -pe 's[ ^ ("UUID=86d58af9-801b-4c25-b59d-80b52b4acc61") ] = "\#\#\#FAULTY_DISK\#\#\#$0";'

The answers above are written in Raku, a member of the Perl-family of programming languages. Once prepended with ###FAULTY_DISK###, a line in the fstab file is ignored during subsequent runs of the same Raku one-liner, i.e. the modification is stable.

See below link for Sample Input and Sample Output.

https://unix.stackexchange.com/a/689556/227738

sed + add string on the beginning of line but ignore if already exist

4 Answers4