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
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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/sh` to explicitly indicate that the script should be executed by `sh`.
- Extended features of other shells like `bash` or `zsh` (e.g., arrays, advanced pattern matching, `[[ ... ]]` conditionals) may not work in `sh`. Therefore, only use POSIX-compliant syntax.
- The `-x` option 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.