107-tricks.md

Shell Scripting Redirection and Exit Status Guide

This guide explains essential shell scripting operators and commands to handle input, output, error redirection, and exit statuses effectively.

File Descriptors Overview

In shell scripting, file descriptors are numeric identifiers that control how data flows in and out of commands. The main file descriptors are:

• 0 (stdin) - Standard input, where commands read input.
• 1 (stdout) - Standard output, where commands print normal output.
• 2 (stderr) - Standard error, where commands print error messages.

Redirection Operators

Redirection operators allow us to control the flow of stdin, stdout, and stderr. Here are the most commonly used redirection operators:

0> - Standard Input (stdin)

Redirects a file as input to a command. Typically, this is used implicitly with <.

command < input.txt       # Redirects input from input.txt to the command
command 0< input.txt      # Equivalent to the above

1> - Standard Output (stdout)

Redirects the standard output of a command to a file.

command > output.txt      # Redirects standard output to output.txt
command 1> output.txt     # Equivalent to the above

2> - Standard Error (stderr)

Redirects the standard error of a command to a file.

command 2> error.log      # Redirects standard error to error.log
&> - Redirect Both stdout and stderr

Combines both stdout and stderr into the same file.

command &> combined.log   # Redirects both output and error to combined.log

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2>&1 - Redirect stderr to stdout

Redirects stderr to wherever stdout is currently directed. This is commonly used to combine outputs.

command > output.log 2>&1 # Redirects output to output.log and error to the same file

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>&2 - Redirect Output to stderr

Redirects any output to stderr, which is helpful for custom error messages.

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echo "An error occurred" >&2   # Sends this message to stderr
&>/dev/null - Suppress Output and Error Messages

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Redirects both stdout and stderr to /dev/null, effectively discarding all output and error messages. This is useful for commands where output is unnecessary.

command &>/dev/null     # Suppresses all output and errors

Exit Status Commands

The exit status of a command is a numeric code representing whether the command ran successfully.

• An exit status of 0 indicates success.
• A non-zero exit status (e.g., 1) indicates an error.
• Exit statuses can be checked to control the flow of a script.

exit 0 - Successful Exit

Exits a script or function with a 0 status, signaling success.

exit 0   # Exits with a status of 0 (success)
exit 1 - #‌ Error Exit

• Exits a script or function with a non-zero status, signaling an error. By convention, exit 1 signifies a general error.

exit 1   # Exits with a status of 1 (error)

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Checking the Exit Status of Commands

The special variable $? stores the exit status of the last executed command, which you can use to check success or failure.

command
if [ $? -eq 0 ]; then
    echo "Command succeeded"
else
    echo "Command failed"
fi

Alternatively, use a direct check:

if command; then
    echo "Command succeeded"
else
    echo "Command failed"
fi

making bash more sane

set -e

set -e - Exit Immediately on Error

By using set -e, a script will exit immediately if any command returns a non-zero exit status, helping catch errors early.

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set -u

set -u

Treat unset variables as an error and exit immediately.

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set -o pipefail

set -o pipefail

If any command in a pipeline fails, the entire pipeline fails. but resume the script execution

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Operator	Description	Example
0>	Redirect stdin	command 0< input.txt
1>	Redirect stdout	command 1> output.txt
2>	Redirect stderr	command 2> error.log
&>	Redirect both stdout and stderr	command &> combined.log
2>&1	Redirect stderr to the same as stdout	command > output.log 2>&1
>&2	Redirect output to stderr	echo "Error message" >&2
&>/dev/null	Discard both output and error messages	command &>/dev/null
exit 0	Exit with success status	exit 0
exit 1	Exit with error status	exit 1