sed

sed: the stream editor is a text editor that performs editing operations on information coming from standard input or a file. Sed edits line-by-line and in a non-interactive way.

Print

sed '' [file]

Print [file] to screen.

  • Why: '' do nothing on the input, so sed just prints whatever the input is.

cat [file] | sed ''

Print [file] to screen.

sed 'p' [file]

Print [file] to screen; but it would print every line twice.

  • Why:

    1. Automatic printing.
    2. -p printing.

  • p is sed's explicit printing command.

sed -n 'p' [file]

Print [file] to screen.

  • -n suppresses the automatic printing.

sed -n '1p' [file]

Print the first line of [file] to screen.

sed -n '1,5' [file]

Print lines (from 1 to 5) of [file] to screen.

sed -n '1,+4p' [file]

Print the first line and the following 4 lines of [file] to screen.

sed -n '1~2p' [file]

Print every other line of [file] to screen.

Delete

Deleting no longer need the -n command because with the delete command, sed will print everything that is not deleted.

sed '1~2d' [file]

Delete every other line starting with the first.

Source file is not being affected. It is still intact. The edits are output to our screen.

If we want to save our edits, we can redirect standard output to a file like so:

sed '1~2d' [file] > everyother.txt

sed -i '1~2d' [file]

-i performs edits in-place.

sed -i.bak '1~2d' [file]

.bak will create a backup file with the .bak extension, and then edit the regular file in-place.

Substitute

Synatx: sed s/old_word/new_word/

  • s is the substitute command.
  • The three slashes (/) are used to separate the different text fields.

By default, the s command operates on the first match in a line and then moves to the next line. Use g flag to change every instance: sed s/old_word/new_word/g.

sed s/old_word/new_word/ [file]

Change the first instance of "old_word" on each line in [file] to "new_word".

sed s/old_word/new_word/g [file]

Change every instance of "old_word" in [file] to "new_word".

sed s/old_word/new_word/2 [file]

Change only the second instances of "old_word" on each line in [file] to "new_word".

sed -n s/old_word/new_word/2p [file]

-n suppressed automatic printing, and p would print lines where substitution took place.

Referencing Matched Text

sed 's/^.*at/REPLACED/' [file]

The wildcard expression matches from the beginning of the line to the last instance of "at". Then the matched text would be substituted by "REPLACED".

sed 's/^.*at/(&)/' [file]

The wildcard expression matches from the beginning of the line to the last instance of "at". Then the matched text would be put in parentheses.

sed 's/\([^ ][^ ]*\) \([^ ][^ ]*\)/\2 \1/' [file]

Switch the first two words of each line.

References

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