Shell

Table of Contents Shells Which shell Mode Portability Test craft Shortcut

Shells

List:

• https://en.wikipedia.org/wiki/Almquist_shell ash - Almquist Shell
• bash - Bourne-Again Shell (a born-again Bourne shell)
• csh - C-Shell
• ksh - Korn Shell
• dash - Debian Almquist Shell (from ash)
• sh - Bourne Shell
• zsh - Zhong Shao Shell

Which shell

List:

• get current interactive shell:
  • echo $0
  • ps --pid "$$"
• get parent shells: ps -o pid,cmd --forest

$ ps -o pid,cmd  --forest
    PID CMD
   5357 /usr/bin/zsh
  17522  \_ zsh -i
  20461      \_ bash
  20781          \_ ps -o pid,cmd --forest
  18106 zsh -i

Configuration

• get installed shells cat /etc/shells
• get default non-interactive shell ls -ltr /bin/sh
• get default interactive shell: echo "$SHELL"
• change default shell: chsh

Mode

Shell can run in:

• interactive mode, so that user can tell dynamically which script to run: bash -i
• non-interactive mode, to execute a single script (which can ask for user input): bash -c 'echo Hello, world!'

Each shell runs in a process

❯ printf "Process %d is running %s" $$ $0
Process 5357 is running zsh%    

❯ zsh -i
❯ printf "Process %d is running %s" $$ $0
Process 17522 is running zsh%                                                                                          ❯ echo $@

❯ bash -c 'printf "Process %d is running %s" $$ $0
Process 15338 is running bash%   

Portability

POSIX is a standard for shell. dash aims at POSIX-compliance. To check your script compatibility, run your script with shellcheck shellcheck --shell=dash ./myscript.sh

Unfortunately, 'portable' is usually a stronger requirement than 'POSIX-compliant' for shell scripts. That is, writing something that runs on any POSIX shell isn't too hard, but getting it to run on any real-world shell is harder.

From here

POSIX non-compliant: - fish

Test

Test framework for shell:

• bats
• shunit2

craft

List:

• consider using strict mode
• use best practice
• lint with spellcheck

Shortcut

Shortcut with Ctrl:

• cancel Y
• interrupt current command C
• clear the line
  • up to the beginning: U
  • forward (right): K
  • downward (left): W
  • several strokes: E U / A K
• go to
  • beginning of the line: A
  • end of the line: E
• clear screen (prompt): L