Reading output of a command into an array in Bash

Question

I need to read the output of a command in my script into an array  The command is  for example   ps aux   grep   grep   x    and it gives the output line by line like this   10 20 30   I need to read the values from the command output into an array  and then I will do some work if the size of the array is less than three

User · Answer

Here is an example  Imagine that you are going to put the files and directory names  under the current folder  to an array and count its items  The script would be like  my array    ls    my array length    my array     echo  my array length  Or  you can iterate over this array by adding the following script  for element in  quot   my array     quot  do    echo  quot   element  quot  done  Please note that this is the core concept and the input is considered to be sanitized before  i e  removing extra characters  handling empty Strings  and etc   which is out of the topic of this thread

User · Answer

You can use  my array      lt command gt       to store the output of command  lt command gt  into the array my array   You can access the length of that array using  my array length    my array       Now the length is stored in my array length

User · Answer

The other answers will break if output of command contains spaces  which is rather frequent  or glob characters like               To get the output of a command in an array  with one line per element  there are essentially 3 ways    With Bash 4 use mapfile   it s the most efficient   mapfile -t my array  lt   lt   my command    Otherwise  a loop reading the output  slower  but safe    my array    while IFS  read -r line  do     my array      line    done  lt   lt   my command    As suggested by Charles Duffy in the comments  thanks    the following might perform better than the loop method in number 2   IFS    n  read -r -d    -a my array  lt   lt   my command  amp  amp  printf   0      Please make sure you use exactly this form  i e   make sure you have the following    IFS    n  on the same line as the read statement  this will only set the environment variable IFS for the read statement only  So it won t affect the rest of your script at all  The purpose of this variable is to tell read to break the stream at the EOL character  n  -r  this is important  It tells read to not interpret the backslashes as escape sequences  -d     please note the space between the -d option and its argument     If you don t leave a space here  the    will never be seen  as it will disappear in the quote removal step when Bash parses the statement  This tells read to stop reading at the nil byte  Some people write it as -d    0   but it is not really necessary  -d    is better  -a my array tells read to populate the array my array while reading the stream  You must use the printf   0  statement after my command  so that read returns 0  it s actually not a big deal if you don t  you ll just get an return code 1  which is okay if you don t use set -e     which you shouldn t anyway   but just bear that in mind  It s cleaner and more semantically correct  Note that this is different from printf     which doesn t output anything  printf   0  prints a null byte  needed by read to happily stop reading there  remember the -d    option         If you can  i e   if you re sure your code will run on Bash 4  use the first method  And you can see it s shorter too   If you want to use read  the loop  method 2  might have an advantage over method 3 if you want to do some processing as the lines are read  you have direct access to it  via the  line variable in the example I gave   and you also have access to the lines already read  via the array   my array     in the example I gave    Note that mapfile provides a way to have a callback eval d on each line read  and in fact you can even tell it to only call this callback every N lines read  have a look at help mapfile and the options -C and -c therein   My opinion about this is that it s a little bit clunky  but can be used sometimes if you only have simple things to do     I don t really understand why this was even implemented in the first place       Now I m going to tell you why the following method   my array      my command      is broken when there are spaces       I m using this command to test    echo  one two   echo  three four  one two three four     Now I m going to use the broken method    my array      echo  one two   echo  three four        declare -p my array declare -a my array    0   one   1   two   2   three   3   four        As you can see  the fields are not the lines       Now look at the correct method    mapfile -t my array  lt   lt  echo  one two   echo  three four     declare -p my array declare -a my array    0   one two   1   three four        Good    Then some people will then recommend using IFS    n  to fix it     IFS    n    my array     echo  one two   echo  three four       declare -p my array declare -a my array    0   one two   1   three four        It works    But now let s use another command  with globs     echo    one two   echo   three four     one two  three four    IFS    n    my array     echo    one two   echo   three four        declare -p my array declare -a my array    0     one two   1   t        What    That s because I have a file called t in the current directory    and this filename is matched by the glob  three four     at this point some people would recommend using set -f to disable globbing  but look at it  you have to change IFS and use set -f to be able to fix a broken technique  and you re not even fixing it really   when doing that we re really fighting against the shell  not working with the shell     mapfile -t my array  lt   lt   echo    one two   echo   three four      declare -p my array declare -a my array    0     one two   1    three four       here we re working with the shell

[arrays] Reading output of a command into an array in Bash

Examples related to arrays

Examples related to bash