2 · Lose Yourself to Scripting

Basics of Bash Scripting

• Execution of commands and control flow expressions
• Variables, control flow, and unique syntax
• Tailored for shell-related tasks

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Variables and Strings in Bash

• Assign with foo=bar
• Access values with $foo
• Differentiating ' and " for strings

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foo=bar
echo "$foo"  # prints bar
echo '$foo'  # prints $foo

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Special Variables in Bash

• $0 - Name of the script
• $1 to $9 - Arguments to the script. $1 is the first argument and so on.
• $@ - All the arguments
• $# - Number of arguments
• $? - Return code of the previous command
• $$ - Process identification number (PID) for the current script
• !! - Entire last command, including arguments. A common pattern is to execute a command only for it to fail due to missing permissions; you can quickly re-execute the command with sudo by doing sudo !!
• $_ - Last argument from the last command. If you are in an interactive shell, you can also quickly get this value by typing Esc followed by . or Alt+.
• Comprehensive list: Special Chars in Bash

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Command Execution and Return Codes

• Conditional execution with &&, ||, ;
• Understanding true and false commands

false || echo "Oops, fail"
# Oops, fail
true || echo "Will not be printed"
#
true && echo "Things went well"
# Things went well
false && echo "Will not be printed"
#
true ; echo "This will always run"
# This will always run
false ; echo "This will always run"
# This will always run

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Permissions – brief recap

Unix-like permissions are:

1. Read (r): Open and read a file. List contents for directories.
2. Write (w): Modify a file’s contents. Add, remove, rename files in directories.
3. Execute (x): Execute a file as a program/script. Access or traverse directories.

Grouped by 3 user categories:

• User (u): The owner of the file.
• Group (g): Users in the file’s group.
• Other (o): Users not the owner or in the file’s group.

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Default permissions

When a file is created in Unix, default permissions are usually:

• Read and Write for the User (rw-)
• Read for the Group (r—)
• Read for Others (r—)

Depicted as: rw-r--r--

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Script Example

• Prefer [[ ]] for comparisons

#!/bin/bash
 
echo "Starting program at $(date)" # Date will be substituted
 
echo "Running program $0 with $# arguments with pid $$"
 
for file in "$@"; do
    grep foobar "$file" > /dev/null 2> /dev/null
    # When pattern is not found, grep has exit status 1
    # We redirect STDOUT and STDERR to a null register since we do not care about them
    if [[ $? -ne 0 ]]; then
        echo "File $file does not have any foobar, adding one"
        echo "# foobar" >> "$file"
    fi
done

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Command substitution using $( CMD )

    GNUPGHOME="$(mktemp -d)" && \
    export GNUPGHOME && \
    for key in \
        "B46DC71E03FEEB7F89D1F2491F7A8F87B9D8F501" \
      ; do \
        gpg --batch --keyserver "keyserver.ubuntu.com" --recv-keys "$key" ; \
    done && \
    gpg --verify "/tmp/$QD_NAME.tar.gz.sha256.asc" "/tmp/$QD_NAME.tar.gz.sha256" && \
    (cd /tmp && sha256sum --check --status "$QD_NAME.tar.gz.sha256") && \

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Control Flow and Functions

• Supports if, case, while, for
• Function example: Creating and entering a directory

mcd () {
    mkdir -p "$1"
    cd "$1"
}

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Shell Functions vs Scripts

• Functions are in the shell’s language; scripts can be in any language.
• Functions load once; scripts load each execution.
• Functions can modify the environment; scripts cannot, but receive exported variables.
• Functions promote code modularity, reuse, and clarity, and are often included in scripts.

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Bash Comparisons and Globbing

• Globbing with wildcards ?, *, and {}

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Shell Tools

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Discovering Command Usage

• Using --help, man command, and TLDR pages
• Example: man rm, tldr tar

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Finding Files

• Tools: find, fd, locate
• Usage scenarios for each

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Finding Code

• grep and alternatives like ack, ag, rg
• Pattern searching examples

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Finding Shell Commands

• Utilizing history, Ctrl+R, fzf bindings
• Autosuggestions and protecting sensitive history entries

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Directory Navigation

• Fast navigation with fasd, autojump
• Advanced tools: tree, broot, nnn, ranger