Exit codes ($?) in Bash Scripting - Step-by-Step Execution

Concept Flow - Exit codes ($?)

Run Command

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Command Finishes

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Shell Stores Exit Code in $?

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Check $? for Success (0) or Failure (non-0)

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Use Exit Code to Decide Next Steps

When a command runs, the shell saves its exit code in $? which tells if it succeeded (0) or failed (non-zero). You can check $? to decide what to do next.

Execution Sample

Bash Scripting

ls /tmp
 echo $?
false
 echo $?

Run 'ls /tmp' and print its exit code, then run 'false' command and print its exit code.

Execution Table

Step	Command Run	Exit Code ($?)	Meaning	Action
1	ls /tmp	0	Success - command found and listed /tmp	Store 0 in $?
2	echo $?	0	Prints last exit code (0)	Output: 0
3	false	1	Failure - false always returns 1	Store 1 in $?
4	echo $?	1	Prints last exit code (1)	Output: 1

💡 After each command, $? stores that command's exit code; 0 means success, non-zero means failure.

Variable Tracker

Variable	Start	After Step 1	After Step 3	Final
$?	undefined	0	1	1

Key Moments - 3 Insights

Why does $? change after each command?

Because $? always holds the exit code of the last command run, as shown in execution_table rows 1 and 3.

What does an exit code of 0 mean?

It means the command succeeded without errors, as seen in execution_table row 1 where ls /tmp returns 0.

Why does the 'false' command return 1?

'false' is a command that always fails and returns exit code 1, shown in execution_table row 3.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of $? after running 'ls /tmp'?

A 0

B 1

C undefined

D ls /tmp

💡 Hint

Check the 'Exit Code ($?)' column in row 1 of the execution_table.

At which step does $? become 1?

A Step 2

B Step 4

C Step 3

D Step 1

💡 Hint

Look at the 'Exit Code ($?)' column and find when it changes to 1.

If the command 'true' was run instead of 'false', what would $? be after that command?

A 2

B 0

C 1

D undefined

💡 Hint

'true' always returns exit code 0, similar to 'ls /tmp' in the table.

Concept Snapshot

Exit codes ($?) in bash store the result of the last command.
0 means success, non-zero means failure.
Check $? immediately after a command to know if it worked.
Use $? in scripts to control flow based on success or failure.

Full Transcript

When you run a command in bash, it finishes and returns an exit code. This code is saved in the special variable $?. A value of 0 means the command succeeded without errors. Any other number means something went wrong. For example, 'ls /tmp' usually succeeds and sets $? to 0. The 'false' command always fails and sets $? to 1. You can check $? right after a command to decide what to do next in your script or terminal session.