Add tack e x on your bash shebang | #!/bin/bash -ex

By | 2014/08/05

Bash scripts can use various options on the shebang (#!/bin/bash). A more common one is: ‘#!/bin/bash -ex’ Here is the scoop!

Here are the -e and -x options:

-e  Exit immediately if a command exits with a non-zero status.
-x  Print commands and their arguments as they are executed.

In short, adding -ex to your #!/bin/bash will give verbose output and also will abort your script immediately if part of the script fails.

Example script in action:

#!/bin/bash -ex
# Default EC2 instances do not have swap. This enables a small swap partition.

if [ $(swapon -s | wc -l) -lt 2 ]; then
  fallocate -l 512M /swap
  mkswap /swap
  echo "/swap swap swap defaults 0 0" >> /etc/fstab
  swapon -a

To see all options available, type the command: help set

$ help set                                               
set: set [-abefhkmnptuvxBCHP] [-o option-name] [--] [arg ...]                                    
    Set or unset values of shell options and positional parameters.                              
    Change the value of shell attributes and positional parameters, or                           
    display the names and values of shell variables.                                             
      -a  Mark variables which are modified or created for export.                               
      -b  Notify of job termination immediately.                                                 
      -e  Exit immediately if a command exits with a non-zero status.                            
      -f  Disable file name generation (globbing).                                               
      -h  Remember the location of commands as they are looked up.                               
      -k  All assignment arguments are placed in the environment for a                           
          command, not just those that precede the command name.                                 
      -m  Job control is enabled.                                                                
      -n  Read commands but do not execute them.                                                 
      -o option-name                                                                             
          Set the variable corresponding to option-name:                                         
              allexport    same as -a                                                            
              braceexpand  same as -B                                                            
              emacs        use an emacs-style line editing interface                             
              errexit      same as -e                                                            
              errtrace     same as -E                                                            
              functrace    same as -T                                                            
              hashall      same as -h                                                            
              histexpand   same as -H                                                            
              history      enable command history                                                
              ignoreeof    the shell will not exit upon reading EOF                              
                           allow comments to appear in interactive commands
              keyword      same as -k
              monitor      same as -m
              noclobber    same as -C
              noexec       same as -n
              noglob       same as -f
              nolog        currently accepted but ignored
              notify       same as -b
              nounset      same as -u
              onecmd       same as -t
              physical     same as -P
              pipefail     the return value of a pipeline is the status of
                           the last command to exit with a non-zero status,
                           or zero if no command exited with a non-zero status
              posix        change the behavior of bash where the default
                           operation differs from the Posix standard to
                           match the standard
              privileged   same as -p
              verbose      same as -v
              vi           use a vi-style line editing interface
              xtrace       same as -x
      -p  Turned on whenever the real and effective user ids do not match.
          Disables processing of the $ENV file and importing of shell
          functions.  Turning this option off causes the effective uid and
          gid to be set to the real uid and gid.
      -t  Exit after reading and executing one command.
      -u  Treat unset variables as an error when substituting.
      -v  Print shell input lines as they are read.
      -x  Print commands and their arguments as they are executed.
      -B  the shell will perform brace expansion
      -C  If set, disallow existing regular files to be overwritten
          by redirection of output.
      -E  If set, the ERR trap is inherited by shell functions.
      -H  Enable ! style history substitution.  This flag is on
          by default when the shell is interactive.
      -P  If set, do not follow symbolic links when executing commands
          such as cd which change the current directory.
      -T  If set, the DEBUG trap is inherited by shell functions.
      --  Assign any remaining arguments to the positional parameters.
          If there are no remaining arguments, the positional parameters
          are unset.
      -   Assign any remaining arguments to the positional parameters.
          The -x and -v options are turned off.
    Using + rather than - causes these flags to be turned off.  The
    flags can also be used upon invocation of the shell.  The current
    set of flags may be found in $-.  The remaining n ARGs are positional
    parameters and are assigned, in order, to $1, $2, .. $n.  If no
    ARGs are given, all shell variables are printed.
    Exit Status:
    Returns success unless an invalid option is given.

Rock on,

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