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?

Solution 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

Solution 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".

---

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

is wrong. It's the same as

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

---

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)

Comments

[Admin]

What are you trying to do, exactly? It's difficult to tell from that one-liner. The first one is correct but unnecessary as cut takes [FILE] as command-line parameter.

[Admin]

Sorry I am trying to cat the results of uptime in seconds. I only want the first field of output from /proc/uptime

[Admin]

Figured it out! I needed to specify the delimiter.... I thought that was set default to space. My one-liner looks like this now: cat /proc/uptime | cut -d' ' -f1

[Admin]

It made sense after I fixed your markdown. :) You can also do this with Awk, and is IMO, easier to remember. awk '{ print $1 }' /proc/uptime

[Admin]

smallo.ruhr.de/award.html

[Aaron Copley]

I am going to create a shell. You'll have to opt-in of course, but every pipe results in a donation to charity from $USER's bank account.

[Admin]

Bonus: How would I then do a if [ output -lt specified time] then do this in bash...

[Amitav Pajni]

@brenguy That would be an entirely separate question. Here we prefer distinct questions to be asked separately, as this is not a forum and not intended to be.