How can remove the last comma separator from a file on Linux?

Example of file :

"is_supported_kafka_ranger" : "true",
"kafka_log_dir" : "/var/log/kafka",
"kafka_pid_dir" : "/var/run/kafka",
"kafka_user" : "kafka",
"kafka_user_nofile_limit" : "128000",
"kafka_user_nproc_limit" : "65536",

expected results:

"is_supported_kafka_ranger" : "true",
"kafka_log_dir" : "/var/log/kafka",
"kafka_pid_dir" : "/var/run/kafka",
"kafka_user" : "kafka",
"kafka_user_nofile_limit" : "128000",
"kafka_user_nproc_limit" : "65536"

Using GNU sed:

sed -i '$s/,$//' file

That is, on the last line ($) substitute (s) the comma at the end of the line (,$) by nothing.

The change will be done in-place due to the -i flag.

With standard sed:

sed '$s/,$//' <file >file.new &&
mv file.new file

Note: Someone suggested an edit to change "on the last line" to "last on the line" (or something similar). This is wrong. When $ is used to specify an address (a line where to apply an editing command), then it refers to the last line of the stream or file. This is different from using $ in a regular expression.


For in-place editing you could use ed - conveniently, it sets the position to the last line by default on opening so you don't need to address the last line explicitly:

ed file

Or, as a one-liner

printf 's/,$//\nwq\n' | ed -s file
  • 2
    In bash you can dispense with printf like this: ed -s file <<< $'s/,$//\nwq\n' (the keyword for search in the bash manual is "here string"). – Ruslan Jun 3 '18 at 18:35

In general, I'd probably go with the straightforward sed solution. However, if your input files are huge, you might want a solution that doesn't take the time to read through the whole file just to edit the last couple of bytes.

If you are 100% sure your input file ends with a comma followed by a newline, you can use truncate to lop off these last two characters, and then re-append a final newline:

filesize=$(stat -L --format "%s" lastcomma.txt)
truncate --size $((filesize - 2)) lastcomma.txt 
echo >> lastcomma.txt 

This assumes a distro that includes GNU truncate or equivalent.

@StéphaneChazelas points out that GNU truncate now supports truncate -s -2 file to shorten the file by two bytes; if you have this version, the above simplifies to:

truncate --size -2 lastcomma.txt
echo >> lastcomma.txt 
  • ...and a filesystem that supports truncate – malat Jun 4 '18 at 8:43
  • @malat, what file system doesn't support truncate? – Stéphane Chazelas Jun 4 '18 at 8:59
  • @StéphaneChazelas Right this is poorly worded. How about: [...] and a filesystem that supports sparse file to have an efficient truncate. The OP describe his solution starting with: " if your input files are huge"... hence my comment. – malat Jun 4 '18 at 9:02
  • 2
    GNU truncate now supports truncate -s -2 file to shorten the file by two bytes. You'd want to use stat -L as for symlinks, you want the size of the target of the symlink (or simply size=$(wc -c < file)). – Stéphane Chazelas Jun 4 '18 at 9:24
  • 2
    @malat, you don't need sparse file support to truncate files. Here, we're trimming data off the end of the file. – Stéphane Chazelas Jun 4 '18 at 9:26

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