[SOLVED] Can I avoid duplicate strings with the sed "a\" command?

Issue

Can I avoid duplicate strings with the sed "a" command?

I added the word "apple" under "true" in my file.txt.

The problem is that every time I run the command "apple" is appended.

$ sed -i '/true/a\apple' file.txt  ...execute 3 time

$ cat file.txt
true
apple
apple
apple

If the word "apple" already exists, repeating the sed command does not want to add any more.

I have no idea, please help me

...

I want to do this,

...execute sed  command anytime

$ cat file.txt
true
apple

Solution

It seems you don't want to append the line apple if the line following the true already contains apple. Then this sed command should do the trick.

sed -i.backup '
/true/!b
$!{N;/\napple$/!s/\n/&apple&/;p;d;}
a\
apple
' file.txt

Explanation of sed commands:

• If the line doesn't contain true then jump to the end of the script, which will print out the line read (/true/!b).

Otherwise the line contains true:

• If it isn't the last line ($!) then
  • read the next line (N).
  • If the next line doesn't consist of apple (/\napple$/!) then insert the apple between two lines (s/\n/&apple&/).
  • Print out the pattern space (p) and start a new cycle (d)

Otherwise it is the last line (and contains true)

• Append apple (a\ apple)

---

Edit:

The above sed script won't work properly if two consecutive true line occurs in the file, as pointed out by @potong. The version below should fix this, if I haven't overlooked something.

sed -i.backup ':a
               /true/!b
               a\
apple
               n
               /^apple$/d
               ba
' file.txt

Explanation:

• /true/!b: If the line doesn't contain true, no further processing is required. Jump to the end of the script. This will print the current pattern space.
• a\ apple: Otherwise, the line contains true. Append apple.
• n: Print the current pattern space and appended line (apple) and replace the pattern space with the next line. This will end the script if no next line available.
• /^apple$/d: If the line read consists of string apple then delete it and start a new cycle (because it is already appended before)
• ba: Jump to the start of the script (label a) without reading an input line.

Answered By - M. Nejat Aydin

Answer Checked By - Terry (WPSolving Volunteer)