Bash conditional based on exit code of command

by

Just remove the brackets:

#!/bin/bash

if ./success.sh; then
    echo "First: success!"
else
    echo "First: failure!"
fi

if ./failure.sh; then
    echo "Second: success!"
else
    echo "Second: failure!"
fi

Explanation: the thing that goes between if and then is a command (or series of commands), the exit status of which is used to determine whether to run the then clause, or the else clause. This is exactly what you want.

So why do people use brackets in if statements? It’s because normally you want to decide which branch of the if to run based on some conditional expression (is "$a" equal to "$b", does a certain file exist, etc). [ is actually a command which parses its arguments as a conditional expression (ignoring the final ]), and then exits with either success or failure depending on whether the conditional is true or false. Essentially, [ ] functions as an adapter that lets you use conditional expressions instead of command success/failure in your if statements. In your case, you want success/failure not a conditional expression, so don’t use the adapter.

BTW, you’ll also sometimes see if [[ some expression ]]; then and if (( some expression )); then. [[ ]] and (( )) are conditional expressions built into bash syntax (unlike [, which is a command). [[ ]] is essentially a better version of [ ] (with some syntax oddities cleaned up and some features added), and (( )) is a somewhat similar construct that does arithmetic expressions.

BTW2 another thing you’ll see in scripts is the exit status being tested by checking the special parameter $?, which gives the exit status of the last command. It looks like this:

somecommand
if [ $? -eq 0 ]; then
    echo "Somecommand: success!"
else
    echo "Somecommand: failure!"
fi

I really consider this cargo cult programming. People are used to seeing [ ] conditional expressions in if statements, and this idiom puts the success test in the form of a conditional expression. But let me run through how it works: it takes the exit status of the command, puts it in a conditional expression, has [ ] evaluate that and turn it right back into an exit status so if can use it. That whole rigamarole is unnecessary; just put the command directly in the if statement.

More Related Contents:

  1. Quick-and-dirty way to ensure only one instance of a shell script is running at a time
  2. Command substitution: backticks or dollar sign / paren enclosed? [duplicate]
  3. How to check the exit status using an ‘if’ statement
  4. Quick-and-dirty way to ensure only one instance of a shell script is running at a time
  5. How does Ctrl-C terminate a child process?
  6. How to kill all subprocesses of shell?
  7. How to pipe multiple commands into a single command in the shell? (sh, bash, …)
  8. How do I check if a directory exists or not in a Bash shell script?
  9. run bash command in new shell and stay in new shell after this command executes
  10. Using groovy, how do you pipe multiple shell commands?
  11. -bash: __git_ps1: command not found
  12. How to wait on all child (and grandchild etc) process spawned by a script
  13. While loop stops reading after the first line in Bash
  14. How can I compare numbers in Bash?
  15. Pipe output and capture exit status in Bash
  16. Redirect stderr and stdout in Bash [duplicate]
  17. Why equal to operator does not work if it is surrounded by space?
  18. Which characters need to be escaped when using Bash?
  19. Capturing Groups From a Grep RegEx
  20. Why does “local” sweep the return code of a command?
  21. bash : Illegal number
  22. Shell redirection i/o order
  23. Using sed to mass rename files
  24. Simulating ENTER keypress in bash script
  25. How to make awk ignore the field delimiter inside double quotes? [duplicate]
  26. Passing arguments by reference
  27. Get current directory or folder name (without the full path)
  28. How do file descriptors work?
  29. Execute a shell function with timeout
  30. Using Bash to display a progress indicator (spinner) [duplicate]

Leave a Comment