Automatically capture output of last command into a variable using Bash

Question

I d like to be able to use the result of the last executed command in a subsequent command  For example     find   -name foo txt   home user some directory foo txt   Now let s say I want to be able to open the file in an editor  or delete it  or do something else with it  e g   mv  lt some-variable-that-contains-the-result gt   some new location   How can I do it  Maybe using some bash variable   Update   To clarify  I don t want to assign things manually  What I m after is something like built-in bash variables  e g   ls  tmp cd         holds the last argument of the previous command  I want something similar  but with the output of the last command   Final update   Seth s answer has worked quite well  Couple of things to bear in mind    don t forget to touch  tmp x when trying the solution for the very first time the result will only be stored if last command s exit code was successful

User · Answer

It can be done using the magic of file descriptors and the lastpipe shell option.

It has to be done with a script - the "lastpipe" option will not work in interactive mode.

Here's the script I've tested with:

$ cat shorttest.sh 
#!/bin/bash
shopt -s lastpipe

exit_tests() {
    EXITMSG="$(cat /proc/self/fd/0)"
}

ls /bloop 2>&1 | exit_tests

echo "My output is \"$EXITMSG\""


$ bash shorttest.sh 
My output is "ls: cannot access '/bloop': No such file or directory"

What I'm doing here is:

setting the shell option shopt -s lastpipe. It will not work without this as you will lose the file descriptor.
making sure my stderr also gets captured with 2>&1
piping the output into a function so that the stdin file descriptor can be referenced.
setting the variable by getting the contents of the /proc/self/fd/0 file descriptor, which is stdin.

I'm using this for capturing errors in a script so if there is a problem with a command I can stop processing the script and exit right away.

shopt -s lastpipe

exit_tests() {
    MYSTUFF="$(cat /proc/self/fd/0)"
    BADLINE=$BASH_LINENO
}

error_msg () {
    echo -e "$0: line $BADLINE\n\t $MYSTUFF"
    exit 1
}

ls /bloop 2>&1 | exit_tests ; [[ "${PIPESTATUS[0]}" == "0" ]] || error_msg

In this way I can add 2>&1 | exit_tests ; [[ "${PIPESTATUS[0]}" == "0" ]] || error_msg behind every command I care to check on.

Now you can enjoy your output!

User · Answer

I don t know of any variable that does this automatically  To do something aside from just copy-pasting the result  you can re-run whatever you just did  eg  vim         Where    is history expansion meaning  the previous command    If you expect there to be a single filename with spaces or other characters in it that might prevent proper argument parsing  quote the result  vim           Leaving it unquoted will allow multiple files to be opened at once as long as they don t include spaces or other shell parsing tokens

User · Answer

Bash is kind of an ugly language   Yes  you can assign the output to variable  MY VAR    find -name foo txt   echo   MY VAR    But better hope your hardest that find only returned one result and that that result didn t have any  odd  characters in it  like carriage returns or line feeds  as they will be silently modified when assigned to a Bash variable   But better be careful to quote your variable correctly when using it   It s better to act on the file directly  e g  with find s -execdir  consult the manual    find -name foo txt -execdir vim            or  find -name foo txt -execdir rename  s   txt   xml

User · Answer

As an alternative to the existing answers  Use while if your file names can contain blank spaces like this   find   -name foo txt   while IFS  read -r var  do   echo   var  done   As I wrote  the difference is only relevant if you have to expect blanks in the file names   NB  the only built-in stuff is not about the output but about the status of the last command

User · Answer

find   -name foo txt 1 gt  tmpfile  amp  amp  mv  cat tmpfile   path to some dir    is yet another way  albeit dirty

User · Answer

This is not strictly a bash solution but you can use piping with sed to get the last row of previous commands output   First lets see what i have in folder  a   rasjani helruo-dhcp022206     find a a a foo a bar a bat a baz rasjani helruo-dhcp022206        Then  your example with ls and cd would turn to sed  amp  piping into something like this   rasjani helruo-dhcp022206     cd  find a  sed    d   rasjani helruo-dhcp022206    a baz  pwd  home rasjani a baz rasjani helruo-dhcp022206    a baz    So  the actual magic happens with sed  you pipe what ever output of what ever command into sed and sed prints the last row which you can use as parameter with back ticks  Or you can combine that to xargs also    man xargs  in shell is your friend

User · Answer

I had a similar need  in which I wanted to use the output of last command into the next one  Much like a    pipe    eg    which gradle   usr bin gradle   ls -alrt  usr bin gradle   to something like -    which gradle    ls -altr      Solution   Created this custom pipe  Really simple  using xargs -    alias    xargs -I      Basically nothing by creating a short hand for xargs  it works like charm  and is really handy  I just add the alias in  bash profile file

User · Answer

Disclamers    This answer is late half a year  D I m a heavy tmux user You have to run your shell in tmux for this to work   When running an interactive shell in tmux  you can easily access the data currently displayed on a terminal  Let s take a look at some interesting commands    tmux capture-pane  this one copies the displayed data to one of the tmux s internal buffers  It can copy the history that s currently not visible  but we re not interested in that now tmux list-buffers  this displays the info about the captured buffers  The newest one will have the number 0  tmux show-buffer -b  buffer num   this prints the contents of the given buffer on a terminal tmux paste-buffer -b  buffer num   this pastes the contents of the given buffer as input   Yeah  this gives us a lot of possibilities now    As for me  I set up a simple alias  alias L  tmux capture-pane  tmux showb -b 0   tail -n 3   head -n 1  and now every time I need to access the last line i simply use   L  to get it   This is independent of the output stream the program uses  be it stdin or stderr   the printing method  ncurses  etc   and the program s exit code - the data just needs to be displayed

User · Answer

I usually do what the others here have suggested     without the assignment    find   -iname    cpp  -print   foo cpp   bar cpp  vi      2 files to edit   You can get fancier if you like    grep -R  some variable      grep -v tags   foo bar xxx   bar foo yyy  vi

User · Answer

I find remembering to pipe the output of my commands into a specific file to be a bit annoying  my solution is a function in my  bash profile that catches the output in a file and returns the result when you need it    The advantage with this one is that you don t have to rerun the whole command  when using find or other long-running commands that can be critical   Simple enough  paste this in your  bash profile    Script    catch stdin  pipe it to stdout and save to a file catch      cat -   tee  tmp catch out    print whatever output was saved to a file res      cat  tmp catch out     Usage    find   -name  filename    catch  path to filename    res  path to filename   At this point  I tend to just add   catch to the end of all of my commands  because there s no cost to doing it and it saves me having to rerun commands that take a long time to finish    Also  if you want to open the file output in a text editor you can do this      vim or whatever your favorite text editor is   vim  lt  res

User · Answer

If all you want is to rerun your last command and get the output  a simple bash variable would work   LAST        So then you can run your command on the output with   yourCommand  LAST   This will spawn a new process and rerun your command  then give you the output  It sounds like what you would really like would be a bash history file for command output  This means you will need to capture the output that bash sends to your terminal  You could write something to watch the  dev or  proc necessary  but that s messy  You could also just create a  special pipe  between your term and bash with a tee command in the middle which redirects to your output file   But both of those are kind of hacky solutions  I think the best thing would be terminator which is a more modern terminal with output logging  Just check your log file for the results of the last command  A bash variable similar to the above would make this even simpler

User · Answer

you can use    1  Example       ls     class1 cpp  class1 h  main cpp  CMakeList txt        rm    1 rm class1 cpp  class1 h  main cpp  CMakeList txt         ls file to remove1 file to remove2 file to remove1 file to remove2      rm    1 rm file to remove1      rm    2 rm file to remove2

User · Answer

You could set up the following alias in your bash profile   alias s  it     history   tail -2   head -1   cut -d    -f4-      Then  by typing  s  after an arbitrary command you can save the result to a shell variable  it     So example usage would be     which python  usr bin python   s   file  it  usr bin python  symbolic link to  python2 6

User · Answer

Capture the output with backticks   output  program arguments  echo  output emacs  output

User · Answer

I just distilled this bash function from the suggestions here   grab            grab           echo  grab     Then  you just do    gt  grab date Do 16  Feb 13 05 04 CET 2012  gt  echo  grab Do 16  Feb 13 05 04 CET 2012   Update  an anonymous user suggested to replace echo by printf   s n  which has the advantage that it doesn t process options like -e in the grabbed text  So  if you expect or experience such peculiarities  consider this suggestion  Another option is to use cat  lt  lt  lt  grab instead

User · Answer

There are more than one ways to do this  One way is to use v   command  which will assign the output of command to v  For example   v   date  echo  v   And you can use backquotes too    v  date  echo  v   From Bash Beginners Guide       When the old-style backquoted form of substitution is used  backslash retains its literal meaning except when followed by           or      The first backticks not preceded by a backslash terminates the command substitution  When using the    COMMAND   form  all characters between the parentheses make up the command  none are treated specially    EDIT  After the edit in the question  it seems that this is not the thing that the OP is looking for  As far as I know  there is no special variable like    for the output of last command

User · Answer

This is a really hacky solution  but it seems to mostly work some of the time   During testing  I noted it sometimes didn t work very well when getting a  C on the command line  though I did tweak it a bit to behave a bit better     This hack is an interactive mode hack only  and I am pretty confident that I would not recommend it to anyone   Background commands are likely to cause even less defined behavior than normal   The other answers are a better way of programmatically getting at results     That being said  here is the  solution      PROMPT COMMAND  LAST   cat  tmp x    exec  gt  dev tty  exec  gt   gt  tee  tmp x     Set this bash environmental variable and issues commands as desired    LAST will usually have the output you are looking for   startide seth gt  fortune Courtship to marriage  as a very witty prologue to a very dull play                  -- William Congreve startide seth gt  echo   LAST  Courtship to marriage  as a very witty prologue to a very dull play                  -- William Congreve

User · Answer

It s quite easy   Use back-quotes   var  find   -name foo txt    And then you can use that any time in the future  echo  var mv  var  somewhere

User · Answer

The shell doesn t have perl-like special symbols that store the echo result of the last command   Learn to use the pipe symbol with awk   find     awk    print  FILE    0      In the example above you could do   find   -name  foo txt    awk    print  mv   0    bar      sh

User · Answer

Here s one way to do it after you ve executed your command and decided that you want to store the result in a variable     find   -name foo txt   home user some directory foo txt   OUTPUT        echo  OUTPUT   home user some directory foo txt   mv  OUTPUT somewhere else    Or if you know ahead of time that you ll want the result in a variable  you can use backticks     OUTPUT  find   -name foo txt    echo  OUTPUT   home user some directory foo txt

User · Answer

I think you might be able to hack out a solution that involves setting your shell to a script containing      bin sh bash   tee  var log bash out log   Then if you set  PROMPT COMMAND to output a delimiter  you can write a helper function  maybe called    that gets you the last chunk of that log  so you can use it like     find lots of files       echo              same output  but doesn t run the command again

User · Answer

By saying  I d like to be able to use the result of the last executed command in a subsequent command   I assume - you mean the result of any command  not just find   If thats the case - xargs is what you are looking for    find   -name foo txt -print0   xargs -0  -I   mv     some new location     OR if you are interested to see the output first   find   -name foo txt -print0       xargs -0  -I   mv     some new location     This command deals with multiple files and works like a charm even if the path and or filename contains space s    Notice the mv     some new location    part of the command  This command is build and executed for each line printed by earlier command  Here the line printed by earlier command is replaced in place of      Excerpt from man page of xargs      xargs - build and execute command lines from standard input   For more detail see man page  man xargs

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