Shell Scripting (Bash)

Bash

Bash (Bourne Again Shell) is a powerful command-line interpreter and scripting language. Bash scripting can significantly enhance productivity by automating repetitive tasks.

Structure of bash

Example of a bash script script.sh:

#!/bin/bash  

# This is a comment
echo "Hello, World!"  # This is an inline comment
echo "Script name: $0"
echo "First argument: $1"
echo "Second argument: $2"

To execute the above script, run ./script.sh arg1 arg2.

Variables

name="Alice"
echo "Hello, $name"

Note that there should be no spaces around the =.

Command Substitution

Whenever you place $( CMD ) it will execute CMD, get the output of the command and substitute it in place. For example:

% echo "We are in $(pwd)"
We are in /Users/zhaosonglin/Desktop/programming/python/numpy
% echo "Today is $(date)"
Today is Tue Jun 25 15:48:09 CST 2024

Exit codes

An exit code is a number returned by a command or script to the calling environment. By convention:

• An exit code of 0 indicates that the command or script executed successfully.
• A non-zero exit code indicates that an error occurred.

Special Variables

• $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.

Special Functions

? and * can be used to match one or any amount of characters respectively.

% ls
01 hstack.py		04 mask.py		basic.ipynb
02 transpose.py		04 slicing.ipynb
03 least-square.py	05 histogram.py
% ls *.ipynb
04 slicing.ipynb	basic.ipynb
% ls b?sic.ipynb
basic.ipynb

{} can be used when a set of strings have common substrings.

% mkdir a b
% touch {a,b}/{x,y,z}

The above command will create three files inside a and b folder (a/x, a/y, a/z, b/x, b/y, b/z) respectively.

mv *{.py,.ipynb} dest
# Will move all *.py and *.sh files to folder dest

Functions

#!/bin/bash

greet() {
    echo "Hello, $1"
}

greet "Alice"
greet "Bob"

If Statement

#!/bin/bash

number=10

if [ $number -gt 5 ]; then
    echo "The number is greater than 5."
else
    echo "The number is 5 or less."
fi

Loops

#!/bin/bash

for item in item1 item2 item3; do
    echo $item
done

#!/bin/bash

counter=1

while [ $counter -le 5 ]; do
    echo "Counter: $counter"
    counter=$((counter + 1))
done

Finding files

find . -name src -type d

• find .: Start searching in the current directory.
• -name src: Match directories named src.
• -type d: Ensure that the match is a directory.

find . -path '*/test/*.py' -type f

• find .: Start searching in the current directory.
• -path ‘/test/.py’: Match files where test appears as a directory in their path and the filename ends with .py (* can substitute any string).
• -type f: Ensure that the match is a regular file (not a directory).

find . -name '*.tmp' -exec rm {} \;

• find .: Start searching in the current directory.
• -name ‘*.tmp’: Match files ends with .tmp.
• -exec rm {} ;: For each file found, execute the rm command to delete it. {} is replaced by the current file’s path.