What does set -e mean in a bash script

Question

I m studying the content of this preinst file that the script executes before that package is unpacked from its Debian archive   deb  file   The script has the following code      bin bash set -e   Automatically added by dh installinit if     1    install    then    if   -d  usr share MyApplicationName    then      echo  MyApplicationName is just installed       return 1    fi    rm -Rf  HOME  config nautilus-actions nautilus-actions conf    rm -Rf  HOME  local share file-manager actions   fi   End automatically added section   My first query is about the line   set -e   I think that the rest of the script is pretty simple  It checks whether the Debian Ubuntu package manager is executing an install operation  If it is  it checks whether my application has just been installed on the system  If it has  the script prints the message  MyApplicationName is just installed  and ends  return 1 mean that ends with an    error     doesn   t it     If the user is asking the Debian Ubuntu package system to install my package  the script also deletes two directories   Is this right or am I missing something

User · Answer

I found this post while trying to figure out what the exit status was for a script that was aborted due to a set -e. The answer didn't appear obvious to me; hence this answer. Basically, set -e aborts the execution of a command (e.g. a shell script) and returns the exit status code of the command that failed (i.e. the inner script, not the outer script).

For example, suppose I have the shell script outer-test.sh:

#!/bin/sh
set -e
./inner-test.sh
exit 62;

The code for inner-test.sh is:

#!/bin/sh
exit 26;

When I run outer-script.sh from the command line, my outer script terminates with the exit code of the inner script:

$ ./outer-test.sh
$ echo $?
26

User · Answer

This is an old question  but none of the answers here discuss the use of set -e aka set -o errexit in Debian package handling scripts  The use of this option is mandatory in these scripts  per Debian policy  the intent is apparently to avoid any possibility of an unhandled error condition   What this means in practice is that you have to understand under what conditions the commands you run could return an error  and handle each of those errors explicitly   Common gotchas are e g  diff  returns an error when there is a difference  and grep  returns an error when there is no match   You can avoid the errors with explicit handling   diff this that      echo   0  there was a difference   gt  amp 2 grep cat food      echo   0  no cat in the food   gt  amp 2    Notice also how we take care to include the current script s name in the message  and writing diagnostic messages to standard error instead of standard output    If no explicit handling is really necessary or useful  explicitly do nothing   diff this that    true grep cat food         The use of the shell s   no-op command is slightly obscure  but fairly commonly seen    Just to reiterate   something    other   is shorthand for  if something  then       nothing else     other fi   i e  we explicitly say other should be run if and only if something fails  The longhand if  and other shell flow control statements like while  until  is also a valid way to handle an error  indeed  if it weren t  shell scripts with set -e could never contain flow control statements    And also  just to be explicit  in the absence of a handler like this  set -e would cause the entire script to immediately fail with an error if diff found a difference  or if grep didn t find a match   On the other hand  some commands don t produce an error exit status when you d want them to  Commonly problematic commands are find  exit status does not reflect whether files were actually found  and sed  exit status won t reveal whether the script received any input or actually performed any commands successfully   A simple guard in some scenarios is to pipe to a command which does scream if there is no output   find things   grep   sed -e  s o me   stuff   grep     It should be noted that the exit status of a pipeline is the exit status of the last command in that pipeline  So the above commands actually completely mask the status of find and sed  and only tell you whether grep finally succeeded    Bash  of course  has set -o pipefail  but Debian package scripts cannot use Bash features  The policy firmly dictates the use of POSIX sh for these scripts  though this was not always the case    In many situations  this is something to separately watch out for when coding defensively  Sometimes you have to e g  go through a temporary file so you can see whether the command which produced that output finished successfully  even when idiom and convenience would otherwise direct you to use a shell pipeline

User · Answer

cat a sh     bin bash   going forward report subshell or command exit value if errors  set -e  cat b txt  echo  quot hi quot     a sh  echo    cat  b txt  No such file or directory hi 0  with set -e commented out we see that echo  quot hi quot  exit status being reported and hi is printed  cat a sh     bin bash   going forward report subshell or command exit value if errors set -e  cat b txt  echo  quot hi quot     a sh  echo    cat  b txt  No such file or directory 1  Now we see b txt error being reported instead and no hi printed  So default behaviour of shell script is to ignore command errors and continue processing and report exit status of last command   If you want to exit on error and report its status we can use -e option

User · Answer

set -e stops the execution of a script if a command or pipeline has an error - which is the opposite of the default shell behaviour  which is to ignore errors in scripts  Type help set in a terminal to see the documentation for this built-in command

User · Answer

I believe the intention is for the script in question to fail fast   To test this yourself  simply type set -e at a bash prompt  Now  try running ls  You ll get a directory listing  Now  type lsd  That command is not recognized and will return an error code  and so your bash prompt will close  due to set -e    Now  to understand this in the context of a  script   use this simple script      bin bash    set -e  lsd   ls   If you run it as is  you ll get the directory listing from the ls on the last line  If you uncomment the set -e and run again  you won t see the directory listing as bash stops processing once it encounters the error from lsd

User · Answer

From help set      -e  Exit immediately if a command exits with a non-zero status    But it s considered bad practice by some  bash FAQ and irc freenode  bash FAQ authors   It s recommended to use   trap  do something  ERR   to run do something function when errors occur   See http   mywiki wooledge org BashFAQ 105

User · Answer

As per bash - The Set Builtin manual  if -e errexit is set  the shell exits immediately if a pipeline consisting of a single simple command  a list or a compound command returns a non-zero status   By default  the exit status of a pipeline is the exit status of the last command in the pipeline  unless the pipefail option is enabled  it s disabled by default    If so  the pipeline s return status of the last  rightmost  command to exit with a non-zero status  or zero if all commands exit successfully   If you d like to execute something on exit  try defining trap  for example   trap onexit EXIT   where onexit is your function to do something on exit  like below which is printing the simple stack trace   onexit    while caller    n      do    done      There is similar option -E errtrace which would trap on ERR instead  e g    trap onerr ERR   Examples  Zero status example     true  echo    0   Non-zero status example     false  echo    1   Negating status examples       false  echo    0   false    true  echo    0   Test with pipefail being disabled     bash -c  set  o pipefail -e  true   true   true  echo success   echo    success 0   bash -c  set  o pipefail -e  false   false   true  echo success   echo    success 0   bash -c  set  o pipefail -e  true   true   false  echo success   echo    1   Test with pipefail being enabled     bash -c  set -o pipefail -e  true   false   true  echo success   echo    1

User · Answer

Script 1  without setting -e    bin bash decho  hi  echo  hello  This will throw error in decho and program continuous to next line  Script 2  With setting -e    bin bash set -e decho  hi   echo  hello    Up to decho  hi  shell will process and program exit  it will not proceed further

[linux] What does set -e mean in a bash script?

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