dynamically run linux shell commands

Question

I have a command that should be executed by a shell script.

Actually the command does not matter the only thing that is important the further command execution and the right escaping of the critical parts.

The command that usually is executed normally in putty is something like this(maybe some additional flags for ls)

rm -r `ls /test/parse_first/ | awk '{print $2}' | grep trash`

but now I have a batch of such command so I would like to execute them in a loop

like

for i in {0..100}
 do
    str=str$i
    ${!str}
 done

where str is :

str0="rm -r `ls /test/parse_first/ | awk '{print $2}' | grep trash`"
str1="rm -r `ls /test/parse_second/ | awk '{print $2}' | grep trash`"

and that gives me a lot of headache cause the execution done by ${!str} brakes the quotations and inline shell between `...` marks

First, it's a bad idea to parse the output of ls. Can you describe the set of files that you want to delete? It looks like you are deleting files with the word trash in the file name. — chepner
– chepner, Commented Sep 19, 2013 at 20:36
cause the output of ls has some rows with two columns where the second one holds the name and the first the timestamp as I said ls should have some flags — JohnRobPeter
– JohnRobPeter, Commented Sep 19, 2013 at 20:59
You should absolutely read mywiki.wooledge.org/BashFAQ/050 aka "I'm trying to put a command in a variable, but the complex cases always fail!" — tripleee
– tripleee, Commented Sep 19, 2013 at 21:31
Provide Short, Self Contained, Correct Example or keep whining on every answer that it is not what you are thinking. Your choice. — Jan Matejka
– Jan Matejka, Commented Sep 19, 2013 at 22:18

Jan Matejka · Accepted Answer · 2013-09-19 20:34:26Z

1

my_rm() { rm -r `ls /test/$1 | awk ... | grep ... `; }
for i in `whatevr`; do
    my_rm $i
done;

answered Sep 19, 2013 at 20:34

Jan Matejka

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3 Comments

JohnRobPeter Over a year ago

No wrapping in a function is not an option cause the commands will be different. But thanks any way.

Jan Matejka Over a year ago

then fix your question. I bet you can still apply the principle of my answer.

JohnRobPeter Over a year ago

yep there is a point in using function wrapping but for now I want to try with string evaluation cause it involves a smaller amount of efort to change the source

user4815162342 · Accepted Answer · 2013-09-19 20:39:53Z

0

Getting this right is surprisingly tricky, but it can be done:

for i in $(seq 0 100)
do
  str=str$i
  eval "eval \"\$$str\""
done

answered Sep 19, 2013 at 20:39

user4815162342

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1 Comment

JohnRobPeter Over a year ago

Thank you actually only this solution fits my needs. Only minor modifications I made to my commands(I've escaped the ` and some $ to avoid the first eval).Thanks a lot.

Jay · Accepted Answer · 2013-09-19 21:03:44Z

0

You can also do:

for i in {0..10}
do
    <whatevercommand>
done

answered Sep 19, 2013 at 21:03

Jay

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2 Comments

JohnRobPeter Over a year ago

looping is not a problem the Main thing is how to dynamically execute a command(by command i mean a long command with params inline shell executions and parameters like the ones in the post descrioption) that is stored in a variable

user4815162342 Over a year ago

@JohnRobPeter Did you try my answer? I tested it with setup like yours (with rm changed to touch) and it worked as expected.

konsolebox · Accepted Answer · 2013-09-19 22:02:51Z

0

It's actually simpler to place them on arrays and use glob patterns:

#!/bin/bash

shopt -s nullglob

DIRS=("/test/parse_first/" "/test/parse_second/")

for D in "${DIRS[@]}"; do
    for T in "$D"/*trash*; do
        rm -r -- "$T"
    done
done

And if rm could accept multiple arguments, you don't need to have an extra loop:

for D in "${DIRS[@]}"; do
    rm -r -- "$D"/*trash*
done

UPDATE:

#!/bin/bash

readarray -t COMMANDS <<'EOF'
rm -r `ls /test/parse_first/ | awk '{print $2}' | grep trash
rm -r `ls /test/parse_second/ | awk '{print $2}' | grep trash
EOF

for C in "${COMMANDS[@]}"; do
    eval "$C"
done

Or you could just read commands from another file:

readarray -t COMMANDS < somefile.txt

edited Sep 19, 2013 at 22:02

answered Sep 19, 2013 at 21:11

konsolebox

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5 Comments

JohnRobPeter Over a year ago

this approach is bounded to rm command but I need a more generic one that would execute any command given by a var

konsolebox Over a year ago

@JohnRobPeter Please see update. And that's the most generic form you can have. Commands can never really be stored on a variable and executed in a safe manner. At least in a truly generic way.

tripleee Over a year ago

I wish you'd show how to change awk '{print $2}' | grep trash to awk '$2 ~ /trash/ { print $2 }'.

konsolebox Over a year ago

@tripleee That's not what he's really after actually. And parsing arguments through word splitting is never a safe way. That should have been obvious to you already.

tripleee Over a year ago

Just a random Useless Use of Grep reaction.

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