How do I set a variable to the output of a command in Bash?

In addition to backticks `command`, command substitution can be done with $(command) or "$(command)", which I find easier to read, and allows for nesting.

OUTPUT=$(ls -1)
echo "${OUTPUT}"

MULTILINE=$(ls \
   -1)
echo "${MULTILINE}"

Quoting (") does matter to preserve multi-line variable values; it is optional on the right-hand side of an assignment, as word splitting is not performed, so OUTPUT=$(ls -1) would work fine.

$(sudo run command)

If you're going to use an apostrophe, you need `, not '. This character is called "backticks" (or "grave accent"):

#!/bin/bash

VAR1="$1"
VAR2="$2"

MOREF=`sudo run command against "$VAR1" | grep name | cut -c7-`

echo "$MOREF"

Some Bash tricks I use to set variables from commands

Sorry, there is a loong answer. But as bash is a shell, where the main goal is to run other unix commands and react on result code and/or output ( commands are often piped filter, etc... ), storing command output in variables is something basic and fundamental.

Therefore, depending on

• compatibility (posix)
• kind of output (filter(s))
• number of variable to set (split or interpret)
• execution time (monitoring)
• error trapping
• repeatability of request (see long running background process, further)
• interactivity (considering user input while reading from another input file descriptor)
• do I miss something?

You could look at showCert function - a complex sample, parsing openssl output for building: 1 associative array for parsing SUBJECT field, 1 standard array for parsing alternatives names and storing dates to UNIXEPOCH. (Using a single fork to date command for converting two dates together) - In How to determine SSL cert expiration date from a PEM certificate?

First simple, old (obsolete), and compatible way

myPi=`echo '4*a(1)' | bc -l`
echo $myPi 
3.14159265358979323844

Compatible, second way

As nesting could become heavy, parenthesis was implemented for this

myPi=$(bc -l <<<'4*a(1)')

Using backticks in script is to be avoided today.

Nested sample:

SysStarted=$(date -d "$(ps ho lstart 1)" +%s)
echo $SysStarted 
1480656334

bash features

Reading more than one variable (with Bashisms)

df -k /
Filesystem     1K-blocks   Used Available Use% Mounted on
/dev/dm-0         999320 529020    401488  57% /

If I just want a used value:

array=($(df -k /))

you could see an array variable:

declare -p array
declare -a array='([0]="Filesystem" [1]="1K-blocks" [2]="Used" [3]="Available" [
4]="Use%" [5]="Mounted" [6]="on" [7]="/dev/dm-0" [8]="999320" [9]="529020" [10]=
"401488" [11]="57%" [12]="/")'

Then:

echo ${array[9]}
529020

But I often use this:

{ read -r _;read -r filesystem size using avail prct mountpoint ; } < <(df -k /)
echo $using
529020

( The first read _ will just drop header line. ) Here, in only one command, you will populate 6 different variables (shown by alphabetical order):

declare -p avail filesystem mountpoint prct size using
declare -- avail="401488"
declare -- filesystem="/dev/dm-0"
declare -- mountpoint="/"
declare -- prct="57%"
declare -- size="999320"
declare -- using="529020"

Or

{ read -a head;varnames=(${head[@]//[K1% -]});varnames=(${head[@]//[K1% -]});
  read ${varnames[@],,} ; } < <(LANG=C df -k /)

Then:

declare -p varnames ${varnames[@],,} 
declare -a varnames=([0]="Filesystem" [1]="blocks" [2]="Used" [3]="Available" [4]="Use" [5]="Mounted" [6]="on")
declare -- filesystem="/dev/dm-0"
declare -- blocks="999320"
declare -- used="529020"
declare -- available="401488"
declare -- use="57%"
declare -- mounted="/"
declare -- on=""

Or even:

{ read _ ; read filesystem dsk[{6,2,9}] prct mountpoint ; } < <(df -k /)
declare -p mountpoint dsk
declare -- mountpoint="/"
declare -a dsk=([2]="529020" [6]="999320" [9]="401488")

(Note Used and Blocks is switched there: read ... dsk[6] dsk[2] dsk[9] ...)

... will work with associative arrays too: read _ disk[total] disk[used] ...

Other related sample: Parsing xrandr output: and end of Firefox tab by bash in a size of x% of display size? or at AskUbuntu.com Parsing xrandr output

Dedicated fd using unnamed fifo:

There is an elegent way! In this sample, I will read /etc/passwd file:

users=()
while IFS=: read -u $list user pass uid gid name home bin ;do
    ((uid>=500)) &&
        printf -v users[uid] "%11d %7d %-20s %s\n" $uid $gid $user $home
done {list}</etc/passwd

Using this way (... read -u $list; ... {list}<inputfile) leave STDIN free for other purposes, like user interaction.

Then

echo -n "${users[@]}"
       1000    1000 user         /home/user
...
      65534   65534 nobody       /nonexistent

and

echo ${!users[@]}
1000 ... 65534

echo -n "${users[1000]}"
      1000    1000 user       /home/user

This could be used with static files or even /dev/tcp/xx.xx.xx.xx/yyy with x for ip address or hostname and y for port number or with the output of a command:

{
    read -u $list -a head          # read header in array `head`
    varnames=(${head[@]//[K1% -]}) # drop illegal chars for variable names
    while read -u $list ${varnames[@],,} ;do
        ((pct=available*100/(available+used),pct<10)) &&
            printf "WARN: FS: %-20s on %-14s %3d <10 (Total: %11u, Use: %7s)\n" \
                "${filesystem#*/mapper/}" "$mounted" $pct $blocks "$use"
     done
 } {list}< <(LANG=C df -k)

And of course with inline documents:

while IFS=\; read -u $list -a myvar ;do
    echo ${myvar[2]}
done {list}<<"eof"
foo;bar;baz
alice;bob;charlie
$cherry;$strawberry;$memberberries
eof

Practical sample parsing CSV files:

As this answer is loong enough, for this paragraph, I just will let you refer to this answer to How to parse a CSV file in Bash?, I read a file by using an unnamed fifo, using syntax like:

exec {FD}<"$file"   # open unnamed fifo for read
IFS=';' read -ru $FD -a headline
while IFS=';' read -ru $FD -a row ;do ...

... But using bash loadable CSV module.

On my website, you may find the same script, reading CSV as inline document.

Sample function for populating some variables:

#!/bin/bash

declare free=0 total=0 used=0 mpnt='??'

getDiskStat() {
    {
        read _
        read _ total used free _ mpnt
    } < <(
        df -k ${1:-/}
    )
}

getDiskStat $1
echo "$mpnt: Tot:$total, used: $used, free: $free."

Nota: declare line is not required, just for readability.

About sudo cmd | grep ... | cut ...

shell=$(cat /etc/passwd | grep $USER | cut -d : -f 7)
echo $shell
/bin/bash

(Please avoid useless cat! So this is just one fork less:

shell=$(grep $USER </etc/passwd | cut -d : -f 7)

All pipes (|) implies forks. Where another process have to be run, accessing disk, libraries calls and so on.

So using sed for sample, will limit subprocess to only one fork:

shell=$(sed </etc/passwd "s/^$USER:.*://p;d")
echo $shell

And with Bashisms:

But for many actions, mostly on small files, Bash could do the job itself:

while IFS=: read -a line ; do
    [ "$line" = "$USER" ] && shell=${line[6]}
  done </etc/passwd
echo $shell
/bin/bash

or

while IFS=: read loginname encpass uid gid fullname home shell;do
    [ "$loginname" = "$USER" ] && break
  done </etc/passwd
echo $shell $loginname ...

Going further about variable splitting...

Have a look at my answer to How do I split a string on a delimiter in Bash?

Alternative: reducing forks by using backgrounded long-running tasks

In order to prevent multiple forks like

myPi=$(bc -l <<<'4*a(1)'
myRay=12
myCirc=$(bc -l <<<" 2 * $myPi * $myRay ")

or to obtain system start time and current shell start time, both as UNIX EPOCH, I could do two nested forks:

myStarted=$(date -d "$(ps ho lstart 1)" +%s)
mySessStart=$(date -d "$(ps ho lstart $$)" +%s)

This work fine, but running many forks is heavy and slow.

And commands like date and bc could make many operations, line by line!!

See:

bc -l <<<$'3*4\n5*6'
12
30

date -f - +%s < <(ps ho lstart 1 $$)
1516030449
1517853288

So building my two variables: $myStarted and $mySessStart could be done in one operation:

{
    read -r myStarted
    read -r mySessStart
} < <(
    date -f - +%s < <(
        ps ho lstart 1 $$
    )
)

could be written on one line:

{ read -r myStarted;read -r mySessStart;}< <(date -f- +%s< <(ps ho lstart 1 $$))

Backgrounded tasks

But we could use a long running background process to make as many request we need, without having to initiate a new fork for each request.

You could have a look how reducing forks make Mandelbrot bash, improve from more than eight hours to less than five seconds.

Under bash, there is a built-in function: coproc:

coproc bc -l
echo 4*3 >&${COPROC[1]}
read -u $COPROC answer
echo $answer
12

echo >&${COPROC[1]} 'pi=4*a(1)'
ray=42.0
printf >&${COPROC[1]} '2*pi*%s\n' $ray
read -u $COPROC answer
echo $answer
263.89378290154263202896

printf >&${COPROC[1]} 'pi*%s^2\n' $ray
read -u $COPROC answer
echo $answer
5541.76944093239527260816

As bc is ready, running in background and I/O are ready too, there is no delay, nothing to load, open, close, before or after operation. Only the operation himself! This become a lot quicker than having to fork to bc for each operation!

The little extra: (Little but powerful!) While bc stay running, they will hold all his registers. So variables or functions could be defined at initialisation step, as first write to ${COPROC[1]}, just after starting the task (... or even at any time).

Into a function newConnector

You may found my newConnector function on GitHub.Com or on my own site (Note on GitHub: there are two files on my site. Function and demo are bundled into one unique file which could be sourced for use or just run for demo.)

Sample:

source shell_connector.sh

tty
/dev/pts/20

ps --tty pts/20 fw
    PID TTY      STAT   TIME COMMAND
  29019 pts/20   Ss     0:00 bash
  30745 pts/20   R+     0:00  \_ ps --tty pts/20 fw

newConnector /usr/bin/bc "-l" '3*4' 12

ps --tty pts/20 fw
    PID TTY      STAT   TIME COMMAND
  29019 pts/20   Ss     0:00 bash
  30944 pts/20   S      0:00  \_ /usr/bin/bc -l
  30952 pts/20   R+     0:00  \_ ps --tty pts/20 fw

declare -p PI
bash: declare: PI: not found

myBc '4*a(1)' PI
declare -p PI
declare -- PI="3.14159265358979323844"

The function myBc lets you use the background task with simple syntax.

Then for date:

newConnector /bin/date '-f - +%s' @0 0
myDate '2000-01-01'
  946681200
myDate "$(ps ho lstart 1)" boottime
myDate now now
read utm idl </proc/uptime
myBc "$now-$boottime" uptime
printf "%s\n" ${utm%%.*} $uptime
  42134906
  42134906

ps --tty pts/20 fw
    PID TTY      STAT   TIME COMMAND
  29019 pts/20   Ss     0:00 bash
  30944 pts/20   S      0:00  \_ /usr/bin/bc -l
  32615 pts/20   S      0:00  \_ /bin/date -f - +%s
   3162 pts/20   R+     0:00  \_ ps --tty pts/20 fw

From there, if you want to end one of background processes, you just have to close its fd:

eval "exec $DATEOUT>&-"
eval "exec $DATEIN>&-"
ps --tty pts/20 fw
    PID TTY      STAT   TIME COMMAND
   4936 pts/20   Ss     0:00 bash
   5256 pts/20   S      0:00  \_ /usr/bin/bc -l
   6358 pts/20   R+     0:00  \_ ps --tty pts/20 fw

which is not needed, because all fd close when the main process finishes.

As they have already indicated to you, you should use `backticks`.

The alternative proposed $(command) works as well, and it also easier to read, but note that it is valid only with Bash or KornShell (and shells derived from those), so if your scripts have to be really portable on various Unix systems, you should prefer the old backticks notation.

I know three ways to do it:

1. Functions are suitable for such tasks:**

   func (){
       ls -l
   }
   

   Invoke it by saying func.

2. Also another suitable solution could be eval:

   var="ls -l"
   eval $var
   

3. The third one is using variables directly:

   var=$(ls -l)
   
       OR
   
   var=`ls -l`
   

You can get the output of the third solution in a good way:

echo "$var"

And also in a nasty way:

echo $var

Just to be different:

MOREF=$(sudo run command against $VAR1 | grep name | cut -c7-)

When setting a variable make sure you have no spaces before and/or after the = sign. I literally spent an hour trying to figure this out, trying all kinds of solutions! This is not cool.

Correct:

WTFF=`echo "stuff"`
echo "Example: $WTFF"

Will Fail with error "stuff: not found" or similar

WTFF= `echo "stuff"`
echo "Example: $WTFF"

If you want to do it with multiline/multiple command/s then you can do this:

output=$( bash <<EOF
# Multiline/multiple command/s
EOF
)

Or:

output=$(
# Multiline/multiple command/s
)

Example:

#!/bin/bash
output="$( bash <<EOF
echo first
echo second
echo third
EOF
)"
echo "$output"

Output:

first
second
third

Using heredoc, you can simplify things pretty easily by breaking down your long single line code into a multiline one. Another example:

output="$( ssh -p $port $user@$domain <<EOF
# Breakdown your long ssh command into multiline here.
EOF
)"

You need to use either

$(command-here)

or

`command-here`

Example

#!/bin/bash

VAR1="$1"
VAR2="$2"

MOREF="$(sudo run command against "$VAR1" | grep name | cut -c7-)"

echo "$MOREF"

If the command that you are trying to execute fails, it would write the output onto the error stream and would then be printed out to the console.

To avoid it, you must redirect the error stream:

result=$(ls -l something_that_does_not_exist 2>&1)

This is another way and is good to use with some text editors that are unable to correctly highlight every intricate code you create:

read -r -d '' str < <(cat somefile.txt)
echo "${#str}"
echo "$str"

Mac/OSX nowadays come with old Bash versions, ie GNU bash, version 3.2.57(1)-release (arm64-apple-darwin21). In this case, one can use:

new_variable="$(some_command)"

A concrete example:

newvar="$(echo $var | tr -d '123')"

Note the (), instead of the usual {} in Bash 4.

You can use backticks (also known as accent graves) or $().

Like:

OUTPUT=$(x+2);
OUTPUT=`x+2`;

Both have the same effect. But OUTPUT=$(x+2) is more readable and the latest one.

Here are two more ways:

Please keep in mind that space is very important in Bash. So, if you want your command to run, use as is without introducing any more spaces.

1. The following assigns harshil to L and then prints it

   L=$"harshil"
   echo "$L"
   

2. The following assigns the output of the command tr to L2. tr is being operated on another variable, L1.

   L2=$(echo "$L1" | tr [:upper:] [:lower:])
   

Some may find this useful. Integer values in variable substitution, where the trick is using $(()) double brackets:

N=3
M=3
COUNT=$N-1
ARR[0]=3
ARR[1]=2
ARR[2]=4
ARR[3]=1

while (( COUNT < ${#ARR[@]} ))
do
  ARR[$COUNT]=$((ARR[COUNT]*M))
  (( COUNT=$COUNT+$N ))
done

If you use bash: For the case you have multiple commands via pipes and need both their results and their exit statuses there is another great solution. It should be clear from the following example:

# This saves all the results into the RES variable
grep -E "\S" "file.txt" | sort | uniq | read -d '' RES
# 'read' exit status 1 means all input was read till EOF, we're OK with that
if (( PIPESTATUS[0] > 1 || PIPESTATUS[1] > 0 || PIPESTATUS[2] > 0 || PIPESTATUS[3] > 1 )); then
  echo "ERROR"
else
  echo "$RES"
fi

In order for this to work you need to enable shopt -s lastpipe (works just for bash AFAIK!), plus if you're in an interactive session you need to disable job control via set +m (not needed in scripts by default).

See this post in another thread for more details.