Reusing output from last command in Bash

Question

Is the output of a Bash command stored in any register  E g  something similar to    capturing the output instead of the exit status   I could assign the output to a variable with    output   command    but that s more typing

User · Answer

The answer is no  Bash doesn t allocate any output to any parameter or any block on its memory  Also  you are only allowed to access Bash by its allowed interface operations  Bash s private data is not accessible unless you hack it

User · Answer

Not sure exactly what you re needing this for  so this answer may not be relevant  You can always save the output of a command  netstat  gt  gt  output txt  but I don t think that s what you re looking for   There are of course programming options though  you could simply get a program to read the text file above after that command is run and associate it with a variable  and in Ruby  my language of choice  you can create a variable out of command output using  backticks     output    ls                          this is a comment  create variable out of command  if output include   Downloads        if statement to see if command includes  Downloads  folder print  there appears to be a folder named downloads in this directory   else print  there is no directory called downloads in this file   end   Stick this in a  rb file and run it  ruby file rb and it will create a variable out of the command and allow you to manipulate it

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You can use -exec to run a command on the output of a command  So it will be a reuse of the output as an example given with a find command below  find   -name anything out -exec rm        you are saying here - gt  find a file called anything out in the current folder  if found  remove it  If it is not found  the remaining after -exec will be skipped

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If you don t want to recompute the previous command you can create a macro that scans the current terminal buffer  tries to guess the  -supposed- output of the last command  copies it to the clipboard and finally types it to the terminal   It can be used for simple commands that return a single line of output  tested on Ubuntu 18 04 with gnome-terminal    Install the following tools  xdootool  xclip   ruby  In gnome-terminal go to Preferences - gt  Shortcuts - gt  Select all and set it to Ctrl shift a   Create the following ruby script   cat  gt   HOME  parse rb  lt  lt EOF    usr bin ruby stdin   STDIN read d   stdin split   n   e   d reverse f   e drop while    item  item         g   f drop while    item  item start with     USER      h   g 0   print h EOF   In the keyboard settings add the following keyboard shortcut   bash  -c   bin sleep 0 3   xdotool key ctrl shift a   xdotool key ctrl shift c      xclip -out      HOME  parse rb    gt   tmp clipboard      cat  tmp clipboard   xclip -sel clip     xdotool key ctrl shift v      The above shortcut    copies the current terminal buffer to the clipboard extracts the output of the last command  only one line  types it into the current terminal

User · Answer

If you are on mac  and don t mind storing your output in the clipboard instead of writing to a variable  you can use pbcopy and pbpaste as a workaround   For example  instead of doing this to find a file and diff its contents with another file     find app -name  one php    var bar app one php    diff  var bar app one php  var bar two php   You could do this     find app -name  one php    pbcopy   diff   pbpaste   var bar two php   The string  var bar app one php is in the clipboard when you run the first command    By the way  pb in pbcopy and pbpaste stand for pasteboard  a synonym for clipboard

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Like konsolebox said  you d have to hack into bash itself  Here is a quite good example on how one might achieve this  The stderred repository  actually meant for coloring stdout  gives instructions on how to build it   I gave it a try  Defining some new file descriptor inside  bashrc like  exec 41 gt  tmp my console log    number is arbitrary  and modify stderred c accordingly so that content also gets written to fd 41  It kind of worked  but contains loads of NUL bytes  weird formattings and is basically binary data  not readable  Maybe someone with good understandings of C could try that out   If so  everything needed to get the last printed line is tail -n 1  logfile

User · Answer

I have an idea that I don t have time to try to implement immediately    But what if you do something like the following      MY HISTORY FILE    get temp filename    MY HISTORY FILE  MY HISTORY FILE bash -i 2 gt  amp 1   tee  MY HISTORY FILE   some command   cat  MY HISTORY FILE      You ll want to filter that down in practice    There might be issues with IO buffering  Also the file might get too huge  One would have to come up with a solution to these problems

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You can use       to recompute  not re-use  the output of the last command   The    on its own executes the last command     echo pierre pierre   echo my name is       echo my name is   echo pierre  my name is pierre

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I think using script command might help  Something like    script -c bash -qf fifo pid Using bash features to set after parsing

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One way of doing that is by using trap DEBUG   f     bash -c   BASH COMMAND   gt  amp   tmp out log    trap  f  DEBUG   Now most recently executed command s stdout and stderr will be available in  tmp out log  Only downside is that it will execute a command twice  once to redirect output and error to  tmp out log and once normally  Probably there is some way to prevent this behavior as well

User · Answer

Yeah  why type extra lines each time  agreed  You can redirect the returned from a command to input by pipeline  but redirecting printed output to input  1 gt  amp 0  is nope  at least not for multiple line outputs  Also you won t want to write a function again and again in each file for the same  So let s try something else  A simple workaround would be to use printf function to store values in a variable  printf -v myoutput  quot  cmd  quot   such as printf -v var  quot  echo ok    echo fine    echo thankyou  quot  echo  quot  var quot    don t forget the backquotes and quotes in either command   Another customizable general solution  I myself use  for running the desired command only once and getting multi-line printed output of the command in an array variable line-by-line  If you are not exporting the files anywhere and intend to use it locally only  you can have Terminal set-up the function declaration  You have to add the function in    bashrc file or in    profile file  In second case  you need to enable Run command as login shell from Edit gt Preferences gt yourProfile gt Command  Make a simple function  say  get prev     preferably pass the commands in quotes  Single commands might still work without          option 1  create an executable with the command s  and run it      echo     gt   tmp exe      bash  tmp exe  gt   tmp out            option 2  if your command is single command  no-pipe  no semi-colons   still it may not run correct in some exceptions       echo   quot    quot    gt   tmp out            option 3   I actually used below      eval  quot    quot   gt   tmp out   or simply  quot    quot   gt   tmp out            return the command s  outputs line by line     IFS   echo -en  quot  n b quot       arr        exec 3 lt  tmp out     while read -u 3 -r line     do         arr    line          echo  line     done     exec 3 lt  amp -    So what we did in option 1 was print the whole command to a temporary file  tmp exe and run it and save the output to another file  tmp out and then read the contents of the  tmp out file line-by-line to an array  Similar in options 2 and 3  except that the commands were exectuted as such  without writing to an executable to be run  In main script   run your command   cmd  quot echo hey ya  echo hey hi  printf  expr 10   10   n    printf    10   20    n  quot  get prev  cmd   or simply get prev  quot echo hey ya  echo hey hi  printf  expr 10   10   n    printf    10   20    n  quot   Now  bash saves the variable even outside previous scope  so the arr variable created in get prev function is accessible even outside the function in the main script   get previous command outputs in arr for  i 0  i lt    arr      i     do     echo   arr i   done   if you re sure that your output won t have escape sequences you bother about  you may simply print the array printf  quot   arr     n quot   Edit  I use the following code in my implementation  get prev         usage                 echo  quot Usage  alphabet   -h   --help             -s   --sep SEP             -v   --var VAR     quot command  quot  quot                 ARGS   getopt -a -n alphabet -o hs v  --long help sep  var  --  quot    quot       if      -ne 0    then usage  return 2  fi     eval set --  ARGS          local var  quot arr quot      IFS   echo -en   n b       for arg in        do         case  arg in             -h --help                  usage                 echo  quot  -h  --help   opens this help quot                  echo  quot  -s  --sep    specify the separator  newline by default quot                  echo  quot  -v  --var    variable name to put result into  arr by default quot                  echo  quot   command     command to execute  Enclose in quotes if multiple lines or pipelines are used  quot                  shift                 return 0                                -s --sep                  shift                 IFS   echo -en  1                  shift                                -v --var                  shift                 var  1                 shift                                - --                  shift                                                   cmd  option                            esac     done     if        -eq 0    then usage  return 1  fi          ERROR      eval  quot    quot   gt   tmp out    2 gt  amp 1       if    ERROR    then echo  ERROR  return 1  fi          local a        exec 3 lt  tmp out     while read -u 3 -r line     do         a    line      done     exec 3 lt  amp -          eval  var      a           print arr  var   comment this to suppress output    print         eval echo     1        print arr         eval printf  quot  s   n quot   quot     1     quot     Ive been using this to print space-separated outputs of multiple pipelined both commands as line separated  get prev -s  quot   quot  -v myarr  quot cmd1   cmd2  cmd3   cmd4 quot   For example  get prev -s     -v myarr whereis python   or  quot whereis python quot    can also be achieved  in this case  by whereis python   tr       n   Now tr command is useful at other places as well  such as echo  PATH   tr       n   But for multiple piped commands    you know now     -Himanshu

User · Answer

Very Simple Solution One that I ve used for years  Script  add to your  bashrc or  bash profile    capture the output of a command so it can be retrieved with ret cap      tee  tmp capture out     return the output of the most recent command that was captured by cap ret      cat  tmp capture out    Usage   find   -name  filename    cap  path to filename    ret  path to filename  I tend to add   cap to the end of all of my commands  This way when I find I want to do text processing on the output of a slow running command I can always retrieve it with res

User · Answer

Inspired by anubhava s answer  which I think is not actually acceptable as it runs each command twice   save output         exec 1 gt  amp 3         -f  tmp current    amp  amp  mv  tmp current  tmp last       exec  gt   gt  tee  tmp current     exec 3 gt  amp 1 trap save output DEBUG   This way the output of last command is in  tmp last and the command is not called twice

[bash] Reusing output from last command in Bash

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