Comment by Xophmeister | Hacker News Reader

Comment by Xophmeister | Hacker News Reader

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Indeed: sed is useful for making small, line-wise tweaks to text. To be honest, I use it rarely (and this is largely because its regexp flavour leaves a lot to be desired). Things like delete the header line[1S] or a simple replacement[2S]. It, like Awk, has some useful line targeting functions (e.g., print lines between two regular expressions[3S], etc.) Awk, on the other hand, is more like a finite state machine for text processing, with the notion of records and fields baked in[4]. You can do the same thing in Awk as in sed (see [*A] references), but it's often easier in sed; vice versa, some things would be impossible or very difficult to do in sed which would be easy in Awk (e.g., [4], which prints the fifth field whenever the first field is "foo"). This doesn't even get into the multiline/statewise stuff you can do in Awk, but the examples would be too big/specific to fit into this comment.

I also learned recently that GNU Awk has networking support[5]. I have no idea why!

[1S] sed '1d'

[1A] awk 'NR!=1 {print}'

[2S] sed 's/foo/bar/'

[2A] awk '{sub(/foo/, "bar")}'

[3S] sed -n '/start_regex/,/end_regex/p'

[3A] awk '/start_regex/,/end_regex/ {print}'

[4] awk '$1=="foo" {print $5}'

[5] https://www.gnu.org/software/gawk/manual/gawkinet/gawkinet.h...

[3A] awk '/start_regex/,/end_regex/ {print}'

can be simplified to:

awk '/start_regex/,/end_regex/'

because in awk, if no action is given, the default action is to print the lines (that match the pattern). And if the pattern is omitted but the action is given, it means do the action on all lines of the input.

Edited to change:

print the line (that matches the pattern)

to

print the lines (that match the pattern)

>[1S] sed '1d'

Similarly

sed 15q

will print only the 1st 15 lines of the input and then terminate. E.g.:

sed 15q file

or

some_command | sed 15q

So, when put in a shell script and then called (with filename arg or using stdin):

sed $1q

is like a specific use of the head command [1]; it prints the first n ($1) lines of the standard input or of the filename argument given - where the value of $1 comes from the first command-line argument passed to the script.

[1] In fact on earlier Unix versions I worked on (which did not have the head command (IIRC), I used to use this sed command in a script called head - similar to tail.

And I also had a script called body :) to complement head and tail, with the appropriate invocation of sed. It takes two command-line arguments ($1 and $2) and prints (only) the lines in that line number range, from the input.

I did not know gawk had networking support I wonder if it could be used on network traffic on the fly. Sort of like irules on an f5. Thank you for sharing!

"sed is useful for making small, line-wise tweaks to text."

Couldn't agree more!

A great example of this was using (surprised it wasn't mentioned) sed with the -i & 's///g' operators while "cleaning" hundreds (seriously) of HTML/PHP files from injected content at a shared hosting provider.

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