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Shell Tools and Scripting

Variables and Operators

  • foo=bar for setting variables, access with $ like $foo
    • No space between = and variable/expression!
  • Strings defined with '' will not substitute variables, but "" will:
    echo "$foo" # bar
    echo '$foo' # $foo
  • In addition to program output via stdout and stderr, programs give a return code - 0 means good, != 0 means an error
  • Important variables:
    • $0 = name of the script
    • $1 to $9 = arguments to the script. $1 is the first argument, etc.
    • $@ = all arguments
    • $# = number of arguments
    • $? = return code of the previous command
    • $$ = process identification number (PID) for the current script (?)
    • !! = entire last command, including arguments
      • If a command fails due to missing permissions, quickly re-execute it with sudo by doing sudo !!
    • $_ = last argument from last command. alt + . does the same thing
  • Logical operators: allows exit codes to be used to conditionally run commands
    • && = “and” operator, e.g. COMMAND_1 && COMMAND_2
    • || = “or” operator, e.g. COMMAND_1 || COMMAND_2
    • ; = same-line command call, no dependency, e.g. COMMAND_1 ; COMMAND_2
    • && and || are short-circuiting, so not all commands will be run if the outcome is determined partway through. These can be combined with true or false, which are programs that always return 0 and 1, respectively
  • $(COMMAND): command substitution, aka output of COMMAND is substituted in
    • Allows you to set variables equal to output, e.g. foo=$(COMMAND)
  • <(COMMAND): process substitution, aka writes command output to temporary file and substitutes file name in, for when a file input is expected

Scripting

  • Shell globbing: using expressions to express multiple values (often command inputs) efficiently
    • ? = represents any one character, e.g. foo? → foo1 foo2 fool, but not foo10
    • * = represents any amount of characters
    • {} = image.{png,jpg} → image.png image.jpg, {a..e} → a b c d e, {1..5} → 1 2 3 4 5
  • Shebang: allows scripts of different types to be run by the shell via ./script{.sh,.py,...}
    • Bash: #!/bin/bash
    • Python: #!/usr/bin/env python (superior to #!/usr/local/bin/python in terms of portability)
  • shellcheck SCRIPT helps with debugging
    • sudo apt install shellcheck
  • Shell functions versus scripts
    • Functions must be in the same language as the shell
    • Functions are loaded once read, scripts are loaded every time they run
    • Functions that are executed will execute the commands in your current shell environment, while running scripts run them in a separate process, i.e. cd will not change your working directory in a script, but will in a function
  • ./SCRIPT vs source SCRIPT: running a script with ./SCRIPT will run it in a new instance of bash, so commands like cd will not affect the working directory in the session, but source SCRIPT will run in the current session
  • Bash scripts are good for simple one-off scripts/a series of commands, Python scripts are good for larger/complex processes

Navigation

  • tldr PROGRAM: gives succinct documentation with examples (web client: https://tldr.inbrowser.app/)
  • find DIR -name "SEARCH_STRING" -type {d,f}: searches recursively inside DIR for directory/file that matches search string
    • Can also search by size, owner, permissions, modified time, etc.
    • Adding -exec COMMAND {} \; will execute command with {} representing the find output becoming COMMAND input
    • fd is an improved alternative: https://github.com/sharkdp/fd#installation
  • locate "SEARCH_STRING": faster by using a database, which is updated using updatedb
    • Can only search by string, and database is not as fresh
  • grep "SEARCH_STRING" FILES: search inside files
    grep -R "SEARCH_STRING": recursive directory search
    COMMAND | grep "SEARCH_STRING": search lines of output from command
  • COMMAND | fzf: from fzf package, allows for an interactive, fuzzy search
    • e.g. find DIR | fzf
  • rg: aka “ripgrep” is a grep alternative with other options to use
    • fd, ag, ack, are all similar grep alternatives
  • Previous command search
    • history NUMBER = spit out the last NUMBER commands in history, can use with | grep
    • ctrl-r = for history search, made more robust with fzf
  • Navigation tools
    • tree: branched visualization of filesystem
    • broot: interactive branched visualization with fuzzy search
    • nnn: provides file system navigation similar to GUI