52

If I have really long output from a command (single line) but I know I only want the first [x] (let's say 8) characters of the output, what's the easiest way to get that? There aren't any delimiters.

74

One way is to use cut:

 command | cut -c1-8

This will give you the first 8 characters of each line of output. Since cut is part of POSIX, it is likely to be on most Unices.

  • 3
    Note that cut -c selects characters; cut -b or head -c selects bytes. This makes a difference in some locales (in practice, when using UTF-8). – Gilles Oct 24 '10 at 22:07
  • You also don't have to specify the start index in this case. Saying cut -c-8 will select from character 1 to 8. – Sparhawk May 9 '14 at 5:08
  • @Steven, cut's equivalent on Windows is? – Pacerier Aug 25 '15 at 13:06
  • Also command | dd bs=8 count=1 2>/dev/null. Not saying it's shorter or superior. Just another alternative. – dubiousjim Sep 24 '15 at 3:50
  • @Gilles, but note that with current versions of GNU cut, cut -c works like cut -b (that is, it doesn't work properly for multi-byte characters). – Stéphane Chazelas Aug 9 '16 at 13:49
23

These are some other ways to get only first 8 characters.

command | head -c8

command | awk '{print substr($0,1,8);exit}' 

command | sed 's/^\(........\).*/\1/;q'

And if you have bash

var=$(command)
echo ${var:0:8}
  • 2
    I think the following sed formulation is a bit easier to read: command | sed 's/\(.\{8\}\).*/\1/' or if your sed supports it: command | sed -r 's/(.{8}).*/\1/'; Otherwise, +1 – Steven D Oct 24 '10 at 4:48
  • Good stuff, but note that head -c counts bytes, not characters. Similarly, among the major Awk implementations, only GNU awk handles multi-byte characters correctly - FreeBSD Awk and Mawk do not. – mklement0 Jul 5 '15 at 17:30
2

Another one liner solution by using parameter expansion

echo ${word:0:x}

EG: word="Hello world"
echo ${word:0:3} or echo ${word::3} 
o/p: Hel


EG.2: word="Hello world"
echo ${word:1:3}
o/p: ell
  • You can also use a variable holding the length, e.g.: x=8; echo ${word:0:$x} instead of hard-coding the integer. – Cometsong Apr 25 at 14:58
1

If you have a sufficiently advanced shell (for example, the following will work in Bash, not sure about dash), you can do:

read -n8 -d$'\0' -r <(command)

After executing read ... <(command), your characters will be in the shell variable REPLY. Type help read to learn about other options.

Explanation: the -n8 argument to read says that we want up to 8 characters. The -d$'\0' says read until a null, rather than to a newline. This way the read will continue for 8 characters even if one of the earlier characters is a newline (but not if its a null). An alternative to -n8 -d$'\0' is to use -N8, which reads for exactly 8 characters or until the stdin reaches EOF. No delimiter is honored. That probably fits your needs better, but I don't know offhand how many shells have a read that honors -N as opposed to honoring -n and -d. Continuing with the explanation: -r says ignore \-escapes, so that, for example, we treat \\ as two characters, rather than as a single \.

Finally, we do read ... <(command) rather than command | read ... because in the second form, the read is executed in a subshell which is then immediately exited, losing the information you just read.

Another option is to do all your processing inside the subshell. For example:

$ echo abcdefghijklm | { read -n8 -d$'\0' -r; printf "REPLY=<%s>\n" "$REPLY"; }
REPLY=<abcdefgh>
  • 1
    If you just want to output the 8 chars, and don't need to process them in the shell, then just use cut. – dubiousjim Sep 8 '12 at 14:04
  • Good to know about read -n <num>; small caveat: Bash 3.x (still current on OS) mistakenly interprets <num> as a byte count and thus fails with multi-byte characters; this has been fixed in Bash 4.x. – mklement0 Jul 6 '15 at 1:41
1

This is portable:

a="$(command)"             # Get the output of the command.
b="????"                   # as many ? as characters are needed.
echo ${a%"${a#${b}}"}      # select that many chars from $a

To build a string of variable length of characters has its own question here.

0

I had this problem when manually generating checksum files in maven repository. Unfortunately cut -c always prints out a newline at the end of output. To suppress that I use xxd:

command | xxd -l$BYTES | xxd -r

It outputs exactly $BYTES bytes, unless the command's output is shorter, then exactly that output.

  • another method to take off cut's trailing newline is to pip it into: | tr -d '\n' – Cometsong Apr 25 at 15:00

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.