How can I loop over the output of a shell command?

user_1171969 0
Viewed 103324

I want to write a script that loops through the output (array possibly?) of a shell command, ps.

Here is the command and the output:

$ ps -ewo pid,cmd,etime | grep python | grep -v grep | grep -v sh
 3089 python /var/www/atm_securit       37:02
17116 python /var/www/atm_securit       00:01
17119 python /var/www/atm_securit       00:01
17122 python /var/www/atm_securit       00:01
17125 python /var/www/atm_securit       00:00

Convert it into bash script (snippet):

for tbl in $(ps -ewo pid,cmd,etime | grep python | grep -v grep | grep -v sh)
do
   echo $tbl
done

But the output becomes:

3089
python
/var/www/atm_securit
38:06
17438
python
/var/www/atm_securit
00:02
17448
python
/var/www/atm_securit
00:01

How do I loop through every row like in the shell output, but in a bash script?

shell linux bash
4 Answers

Here is another bash-based solution, inspired by comment of @Gordon Davisson.

For this we need (atleast bash v1.13.5 (1992) or later verison), because Process-Substitution2,3,4 while read var; do { ... }; done < <(...);, etc are used.

#!/bin/bash
while IFS= read -a oL ; do {  # reads single/one line
    echo "${oL}";  # prints that single/one line
};
done < <(ps -ewo pid,cmd,etime | grep python | grep -v grep | grep -v sh);
unset oL;

Note: You can use any simple or complex command/command-set inside the <(...) which may have multiple output lines.
And what code does what function is shown here.

And here is a single/one-liner way:
while IFS= read -a oL ; do { echo "${oL}"; }; done < <(ps -ewo pid,cmd,etime | grep python | grep -v grep | grep -v sh); unset oL;

( As Process-Substitution is not part of POSIX yet So its not supported in many POSIX compliant shell or in POSIX shell mode of bash-shell. Process-Substitution existed in bash since 1992 (so that is 28yrs ago from now/2020), & existed in ksh86 (before 1985)1. So POSIX should've included it. )
If you or any user wants to use something similar as Process-Substitution in POSIX compliant shell (i.e: sh, ash, dash, pdksh/mksh, etc), then look into NamedPipes.

user_35538080
Related