Bash

How to set a variable to the output of a command in Bash? [Answered]

- by ᴾᴿᴼᵍʳᵃᵐᵐᵉʳ

Query explained:

I have a pretty simple script that is something like the following:

#!/bin/bash

VAR1="$1"
MOREF='sudo run command against $VAR1 | grep name | cut -c7-'

echo $MOREF

When I run this script from the command line and pass it the arguments, I am not getting any output. However, when I run the commands contained within the $MOREF variable, I am able to get output.

How can one take the results of a command that needs to be run within a script, save it to a variable, and then output that variable on the screen?

How to set a variable to the output of a command in Bash? Answer #1:

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.

Answer #2:

$(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"

Answer #3:

Some Bash tricks I use to set variables from commands

Sorry, there is a long answer, but as bash is a shell, where the main goal is to run other unix commands and react to 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?

First simple, old, and compatible way

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

Mostly compatible, second way

As nesting could become heavy, parenthesis was implemented for this

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

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 foo ; 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 foo disk[total] disk[used] ...

Dedicated fd using unnamed fifo:

There is an elegant 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

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 ...

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

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

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!

Border effect: While bc stay running, they will hold all registers, so some variables or functions could be defined at initialisation step, as first write to ${COPROC[1]}, just after starting the task (via coproc).

Into a function newConnector

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 the 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.

Answer #4:

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.

Answer #5:

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

How to set a variable to the output of a command in Bash? Answer #6:

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"

Answer #7:

Just to be different:

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

Hope you learned something from this post.

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About ᴾᴿᴼᵍʳᵃᵐᵐᵉʳ

Linux and Python enthusiast, in love with open source since 2014, Writer at programming-articles.com, India.

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