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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.

0
130

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.

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  • 5
    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 'SO- stop being evil' Oct 24 '10 at 22:07
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    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
37

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}
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    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
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    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
11

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
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    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 '19 at 14:58
  • worth noting this will not be possible in ksh88, only 93 – access_granted Apr 29 '20 at 1:45
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    @Cometsong Testing with the Bash shell that came with "Git for Windows", it looks like you don't need to prefix x with the $ sign in this case: x=8; echo ${word:0:x} will work the same. – AJM-Reinstate-Monica Mar 26 at 11:10
4

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>
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    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
  • This is a great and useful answer. Much more general than the others. – not2qubit Oct 25 '19 at 10:08
2

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.

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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.

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  • another method to take off cut's trailing newline is to pip it into: | tr -d '\n' – Cometsong Apr 25 '19 at 15:00
0

How to consider Unicode + UTF-8

Let's do a quick test for those interested in Unicode characters rather than just bytes. Each character of áéíóú (acute accented vowels) is made up of two bytes in UTF-8. With:

printf 'áéíóú' | LC_CTYPE=en_US.UTF-8 awk '{print substr($0,1,3);exit}'
printf 'áéíóú' | LC_CTYPE=C awk '{print substr($0,1,3);exit}'
printf 'áéíóú' | LC_CTYPE=en_US.UTF-8 head -c3
echo
printf 'áéíóú' | LC_CTYPE=C head -c3

we get:

áéí
á
á
á

so we see that only awk + LC_CTYPE=en_US.UTF-8 considered the UTF-8 characters. The other approaches took only three bytes. We can confirm that with:

printf 'áéíóú' | LC_CTYPE=C head -c3 | hd

which gives:

00000000  c3 a1 c3                                          |...|
00000003

and the c3 by itself is trash, and does not show up on the terminal, so we saw only á.

awk + LC_CTYPE=en_US.UTF-8 actually returns 6 bytes however.

We could also have equivalently tested with:

printf '\xc3\xa1\xc3\xa9\xc3\xad\xc3\xb3\xc3\xba' | LC_CTYPE=en_US.UTF-8 awk '{print substr($0,1,3);exit}'

and if you want a general parameter:

n=3
printf 'áéíóú' | LC_CTYPE=en_US.UTF-8 awk "{print substr(\$0,1,$n);exit}"

Question more specific about Unicode + UTF-8: https://superuser.com/questions/450303/unix-tool-to-output-first-n-characters-in-an-utf-8-encoded-file

Related: https://stackoverflow.com/questions/1405611/how-to-extract-the-first-two-characters-of-a-string-in-shell-scripting

Tested on Ubuntu 21.04.

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