Overview
`sh` is one of the default shells on Unix and Linux systems, used for script execution and interactive command processing. Its strict adherence to the POSIX standard makes scripts written for `sh` highly portable across various systems.
Key Features
- POSIX Compatibility: Provides a foundation for writing standard shell scripts.
- Script Execution: Runs sequences of commands stored in files.
- Interactive Command Interpretation: Processes commands directly entered by the user.
- System Default: Typically pre-installed on most Unix/Linux systems.
Key Options
`sh` offers various options that affect script execution behavior and debugging.
Script Execution and Debugging
Generated command:
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Description:
`sh` Executes the command.
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Usage Examples
Executing a Script File
sh my_script.shExecutes a specified shell script file using `sh`.
Executing a Command String Directly
sh -c "echo 'Hello from sh!' && ls -l"Uses the `-c` option to execute a quoted command string with `sh`.
Executing a Script in Debug Mode
sh -x debug_script.shUses the `-x` option to trace script execution for debugging purposes.
Executing Commands from Standard Input
echo "ls -a" | shExecutes commands passed via a pipe using `sh`.
Exiting Immediately on Error
sh -e error_prone_script.shEnsures the script terminates immediately if an error occurs.
Tips & Notes
`sh` is a powerful tool, but it's important to understand its differences from other shells.
When Writing Scripts
- It's good practice to start scripts with
#!/bin/shto explicitly indicate that the script should be executed bysh. - Extended features of other shells like
bashorzsh(e.g., arrays, advanced pattern matching,[[ ... ]]conditionals) may not work insh. Therefore, only use POSIX-compliant syntax. - The
-xoption is very useful for debugging, helping to understand script flow and variable values.
Compatibility
On most Linux distributions, /bin/sh is a symbolic link to dash (Debian/Ubuntu family) or bash (Red Hat/Fedora family). Consequently, sh scripts run in the POSIX-compliant mode of the linked shell. This means they might behave differently from bash scripts.