How can I store a command in a variable in a shell script?

Do not use eval! It has a major risk of introducing arbitrary code execution.

BashFAQ-50 - I'm trying to put a command in a variable, but the complex cases always fail.

Put it in an array and expand all the words with double-quotes "${arr[@]}" to not let the IFS split the words due to Word Splitting.

cmdArgs=()
cmdArgs=('date' '+%H:%M:%S')

and see the contents of the array inside. The declare -p allows you see the contents of the array inside with each command parameter in separate indices. If one such argument contains spaces, quoting inside while adding to the array will prevent it from getting split due to Word-Splitting.

declare -p cmdArgs
declare -a cmdArgs='([0]="date" [1]="+%H:%M:%S")'

and execute the commands as

"${cmdArgs[@]}"
23:15:18

(or) altogether use a bash function to run the command,

cmd() {
   date '+%H:%M:%S'
}

and call the function as just

cmd

POSIX sh has no arrays, so the closest you can come is to build up a list of elements in the positional parameters. Here's a POSIX sh way to run a mail program

# POSIX sh
# Usage: sendto subject address [address ...]
sendto() {
    subject=$1
    shift
    first=1
    for addr; do
        if [ "$first" = 1 ]; then set --; first=0; fi
        set -- "$@" --recipient="$addr"
    done
    if [ "$first" = 1 ]; then
        echo "usage: sendto subject address [address ...]"
        return 1
    fi
    MailTool --subject="$subject" "$@"
}

Note that this approach can only handle simple commands with no redirections. It can't handle redirections, pipelines, for/while loops, if statements, etc

Another common use case is when running curl with multiple header fields and payload. You can always define args like below and invoke curl on the expanded array content

curlArgs=('-H' "keyheader: value" '-H' "2ndkeyheader: 2ndvalue")
curl "${curlArgs[@]}"

Another example,

payload='{}'
hostURL='http://google.com'
authToken='someToken'
authHeader='Authorization:Bearer "'"$authToken"'"'

now that variables are defined, use an array to store your command args

curlCMD=(-X POST "$hostURL" --data "$payload" -H "Content-Type:application/json" -H "$authHeader")

and now do a proper quoted expansion

curl "${curlCMD[@]}"

var=$(echo "asdf")
echo $var
# => asdf

Using this method, the command is immediately evaluated and its return value is stored.

stored_date=$(date)
echo $stored_date
# => Thu Jan 15 10:57:16 EST 2015
# (wait a few seconds)
echo $stored_date
# => Thu Jan 15 10:57:16 EST 2015

The same with backtick

stored_date=`date`
echo $stored_date
# => Thu Jan 15 11:02:19 EST 2015
# (wait a few seconds)
echo $stored_date
# => Thu Jan 15 11:02:19 EST 2015

Using eval in the $(...) will not make it evaluated later:

stored_date=$(eval "date")
echo $stored_date
# => Thu Jan 15 11:05:30 EST 2015
# (wait a few seconds)
echo $stored_date
# => Thu Jan 15 11:05:30 EST 2015

Using eval, it is evaluated when eval is used:

stored_date="date" # < storing the command itself
echo $(eval "$stored_date")
# => Thu Jan 15 11:07:05 EST 2015
# (wait a few seconds)
echo $(eval "$stored_date")
# => Thu Jan 15 11:07:16 EST 2015
#                     ^^ Time changed

In the above example, if you need to run a command with arguments, put them in the string you are storing:

stored_date="date -u"
# ...

For Bash scripts this is rarely relevant, but one last note. Be careful with eval. Eval only strings you control, never strings coming from an untrusted user or built from untrusted user input.

For bash, store your command like this:

command="ls | grep -c '^'"

Run your command like this:

echo $command | bash

Not sure why so many answers make it complicated! use alias [command] 'string to execute' example:

alias dir='ls -l'

./dir
[pretty list of files]

I faced this problem with the following command:

awk '{printf "%s[%s]\n", $1, $3}' "input.txt"

I need to build this command dynamically:

The target file name input.txt is dynamic and may contain space.

The awk script inside {} braces printf "%s[%s]\n", $1, $3 is dynamic.

Challenge:

1. Avoid extensive quote escaping logic if there are many " inside the awk script.
2. Avoid parameter expansion for every $ field variable.

The solutions bellow with eval command and associative arrays do not work. Due to bash variable expansions and quoting.

Solution:

Build bash variable dynamically, avoid bash expansions, use printf template.

 # dynamic variables, values change at runtime.
 input="input file 1.txt"
 awk_script='printf "%s[%s]\n" ,$1 ,$3'

 # static command template, preventing double-quote escapes and avoid variable  expansions.
 awk_command=$(printf "awk '{%s}' \"%s\"\n" "$awk_script" "$input")
 echo "awk_command=$awk_command"

 awk_command=awk '{printf "%s[%s]\n" ,$1 ,$3}' "input file 1.txt"

Executing variable command:

bash -c "$awk_command"

Alternative that also works

bash << $awk_command

I tried various different methods:

printexec() {
  printf -- "\033[1;37m$\033[0m"
  printf -- " %q" "$@"
  printf -- "\n"
  eval -- "$@"
  eval -- "$*"
  "$@"
  "$*"
}

Output:

$ printexec echo  -e "foo\n" bar
$ echo -e foo\\n bar
foon bar
foon bar
foo
 bar
bash: echo -e foo\n bar: command not found

As you can see, only the third one, "$@" gave the correct result.

#!/bin/bash
#Note: this script works only when u use Bash. So, don't remove the first line.

TUNECOUNT=$(ifconfig |grep -c -o tune0) #Some command with "Grep".
echo $TUNECOUNT                         #This will return 0 
                                    #if you don't have tune0 interface.
                                    #Or count of installed tune0 interfaces.

Be careful registering an order with the: X=$(Command)

This one is still executed. Even before being called. To check and confirm this, you can do:

echo test;
X=$(for ((c=0; c<=5; c++)); do
sleep 2;
done);
echo note the 5 seconds elapsed

First of all, there are functions for this. But if you prefer variables then your task can be done like this:

$ cmd=ls

$ $cmd # works
file  file2  test

$ cmd='ls | grep file'

$ $cmd # not works
ls: cannot access '|': No such file or directory
ls: cannot access 'grep': No such file or directory
 file

$ bash -c $cmd # works
file  file2  test

$ bash -c "$cmd" # also works
file
file2

$ bash <<< $cmd
file
file2

$ bash <<< "$cmd"
file
file2

Or via a temporary file

$ tmp=$(mktemp)
$ echo "$cmd" > "$tmp"
$ chmod +x "$tmp"
$ "$tmp"
file
file2

$ rm "$tmp"

As you don't specify any scripting language, I would recommand tcl, the Tool Command Language for this kind of purpose.

Then in the first line, add the appropriate shebang:

#!/usr/local/bin/tclsh

with appropriate location you can retrieve with which tclsh.

In tcl scripts, you can call operating system commands with exec.