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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.

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3 Answers

up vote 11 down vote accepted

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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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 at 5:08
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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

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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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"; }
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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
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