How to pipe output from cat to cut

Question

I want to cat /proc/uptime into cut -f1 in a Bash script.

I've tried;

cat /proc/uptime | cut -f1
cat /proc/uptime > cut -f1
cut -f1 < cat /proc/uptime

Do I need to use echo or something else to make this happen?

Answer 1

The default field delimiter for cut is a tab. Since your file has a space instead, you need to specify the delimiter:

-d ' '

And you really don't need to use cat or a pipe at all. Just read the file directly.

cut -f 1 -d ' ' /proc/uptime

Answer 2

cat /proc/uptime | cut -f1 -d' '

is correct

< /proc/uptime  cut -f1 -d' '

is correct and more efficient as it reads from /proc/uptime directly without creating a pipe (not that it matters here much).

It's generally advisable to use the second form on forums, or else you'll get purists coming after you shouting "useless use of cat".

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cut -f1 -d' ' < cat /proc/uptime

is wrong. It's the same as

cut -f1 -d' ' /proc/uptime < cat

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If you're in bash, you can also use <():

cut -f1 -d' ' < <(cat /proc/uptime)

This creates an anonymous named pipe for reading and the output of cat /proc/uptime will be piped into it. But again—useless use of cat.

Other than that, cut can also take a file argument so all the redirect versions will also work without the < (it shouldn't matter efficiency-wise):

cut -f1 -d' '/proc/uptime

Or with the <() pipe:

cut -f1 -d' ' <(cat /proc/uptime)

Answer 3

By default the delimiter for cut is TAB, you could change it to be whitespace with

cat /proc/uptime | cut -f1 -d " " 

Although in those cases I prefer to use awk:

cat /proc/uptime | awk '{print$1}'