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

Question

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

User · Answer

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    The following assigns harshil to L and then prints it  L   harshil  echo   L   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

User · Answer

Just to be different   MOREF   sudo run command against  VAR1   grep name   cut -c7-

User · Answer

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  lt   lt  cat somefile txt  echo     str   echo   str

User · Answer

sudo run command    If you re going to use an apostrophe  you need    not    This character is called  quot backticks quot   or  quot grave accent quot       bin bash  VAR1  quot  1 quot  VAR2  quot  2 quot   MOREF  sudo run command against  quot  VAR1 quot    grep name   cut -c7-   echo  quot  MOREF quot

User · Answer

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  quot stuff quot   echo  quot Example   WTFF quot   Will Fail with error  quot stuff  not found quot  or similar WTFF   echo  quot stuff quot   echo  quot Example   WTFF quot

User · Answer

If you want to do it with multiline multiple command s then you can do this   output    bash  lt  lt EOF   Multiline multiple command s EOF     Or   output      Multiline multiple command s     Example      bin bash output     bash  lt  lt 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  lt  lt EOF   Breakdown your long ssh command into multiline here  EOF

User · Answer

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

User · Answer

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

User · Answer

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 gt  amp 1

User · Answer

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 to resut 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  lt  lt  lt  4 a 1     Nested sample  SysStarted   date -d  quot   ps ho lstart 1  quot    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   quot Filesystem quot   1   quot 1K-blocks quot   2   quot Used quot   3   quot Available quot    4   quot Use  quot   5   quot Mounted quot   6   quot on quot   7   quot  dev dm-0 quot   8   quot 999320 quot   9   quot 529020 quot   10    quot 401488 quot   11   quot 57  quot   12   quot   quot     Then  echo   array 9   529020  But I often use this    read foo   read filesystem size using avail prct mountpoint      lt   lt  df -k    echo  using 529020  The first read foo will just skip header line  but in only one command  you will populate 7 different variables  declare -p avail filesystem foo mountpoint prct size using declare -- avail  quot 401488 quot  declare -- filesystem  quot  dev dm-0 quot  declare -- foo  quot Filesystem     1K-blocks   Used Available Use  Mounted on quot  declare -- mountpoint  quot   quot  declare -- prct  quot 57  quot  declare -- size  quot 999320 quot  declare -- using  quot 529020 quot   Or   read -a head varnames    head      K1  -    varnames    head      K1  -       read   varnames            lt   lt  LANG C df -k     Then  declare -p varnames   varnames        declare -a varnames   0   quot Filesystem quot   1   quot blocks quot   2   quot Used quot   3   quot Available quot   4   quot Use quot   5   quot Mounted quot   6   quot on quot   declare -- filesystem  quot  dev dm-0 quot  declare -- blocks  quot 999320 quot  declare -- used  quot 529020 quot  declare -- available  quot 401488 quot  declare -- use  quot 57  quot  declare -- mounted  quot   quot  declare -- on  quot  quot   Or even    read foo   read filesystem dsk  6 2 9   prct mountpoint      lt   lt  df -k    declare -p mountpoint dsk declare -- mountpoint  quot   quot  declare -a dsk   2   quot 529020 quot   6   quot 999320 quot   9   quot 401488 quot     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 elegent way  users    while IFS   read -u  list user pass uid gid name home bin  do       uid gt  500    amp  amp          printf -v users uid   quot  11d  7d  -20s  s n quot   uid  gid  user  home done  list  lt  etc passwd  Using this way      read -u  list       list  lt inputfile  leave STDIN free for other purposes  like user interaction  Then echo -n  quot   users     quot         1000    1000 user          home user           65534   65534 nobody        nonexistent  and echo    users     1000     65534  echo -n  quot   users 1000   quot        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        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 lt 10    amp  amp              printf  quot WARN  FS   -20s on  -14s  3d  lt 10  Total   11u  Use   7s  n quot                     quot   filesystem   mapper   quot   quot  mounted quot   pct  blocks  quot  use quot       done     list  lt   lt  LANG C df -k   And of course with inline documents  while IFS    read -u  list -a myvar  do     echo   myvar 2   done  list  lt  lt  quot eof quot  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  getDiskStat         local foo               read foo         read foo total used free foo        lt   lt           df -k   1 -            getDiskStat  1 echo  total  used  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  lt  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  lt  etc passwd  quot s   USER      p d quot   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        quot  line quot     quot  USER quot     amp  amp  shell   line 6     done  lt  etc passwd echo  shell  bin bash  or while IFS   read loginname encpass uid gid fullname home shell do        quot  loginname quot     quot  USER quot     amp  amp  break   done  lt  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  lt  lt  lt  4 a 1   myRay 12 myCirc   bc -l  lt  lt  lt  quot  2    myPi    myRay  quot    or myStarted   date -d  quot   ps ho lstart 1  quot    s  mySessStart   date -d  quot   ps ho lstart     quot    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  lt  lt  lt   3 4 n5 6  12 30  date -f -   s  lt   lt  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  gt  amp   COPROC 1   read -u  COPROC answer echo  answer 12  echo  gt  amp   COPROC 1    pi 4 a 1   ray 42 0 printf  gt  amp   COPROC 1    2 pi  s n   ray read -u  COPROC answer echo  answer 263 89378290154263202896  printf  gt  amp   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 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 uniq 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  quot -l quot   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  quot 3 14159265358979323844 quot   The function myBc lets you use the background task with simple syntax  and for date  newConnector  bin date  -f -   s   0 0 myDate  2000-01-01    946681200 myDate  quot   ps ho lstart 1  quot  boottime myDate now now   read utm idl  lt  proc uptime myBc  quot  now- boottime quot  uptime printf  quot  s n quot    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  quot exec  DATEOUT gt  amp - quot  eval  quot exec  DATEIN gt  amp - quot  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

User · Answer

I know three ways to do it    Functions are suitable for such tasks     func         ls -l     Invoke it by saying func  Also another suitable solution could be eval   var  ls -l  eval  var  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

User · Answer

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  lt     ARR        do   ARR  COUNT     ARR COUNT  M        COUNT  COUNT  N    done

User · Answer

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

User · Answer

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

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

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