awk print does not work in bash -c

Question

I noticed that this works fine (will print only the PID of unattended-upgrades process):

lslocks | awk '/unattended-upgrades/ { print $2 }'

But this effectively ignores the print part (will print the entire line):

bash -c "lslocks | awk '/unattended-upgrades/ { print $2 }'"

This is an Ubuntu 18.04 and Bash version 4.4.20.

What is spacial about bash -c that causes this?

steeldriver · Accepted Answer · 2022-03-18T12:40:16.553

6

There's nothing special about bash -c here - it's the fact that the outer double quotes permit expansion of $2 by the interactive shell:

$ set -x
$ bash -c "echo foo bar | awk '{ print $2 }'"
+ bash -c 'echo foo bar | awk '\''{ print  }'\'''
foo bar

(The "gotcha" here is that single quotes are not special inside double quotes; the outer double quotes "preserve their literal value". See Bash Reference Manual 3.1.2.3 Double Quotes.)

Compare:

$ bash -c "echo foo bar | awk '{ print \$2 }'"
+ bash -c 'echo foo bar | awk '\''{ print $2 }'\'''
bar

edited Mar 18 '22 at 12:40

answered Mar 18 '22 at 01:52

steeldriver

81,074

Ouch. That hurts. – yujaiyu Mar 18 '22 at 01:59
3

@foki quoting and escaping in shell are always a pain. It's something you just have to get used to and learn how to deal with...and it gets worse if you need multiple nested levels of quoting (e.g. with bash -c, ssh, and many others) as in your Q. See Why does my shell script choke on whitespace or other special characters? and be sure to follow the link to the quoting tag. – cas Mar 18 '22 at 03:46
@cas Thank you for the references, they are very useful. – yujaiyu Mar 18 '22 at 05:32

awk print does not work in bash -c

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