Bash Programming 筆記

Beginning

Bash shebang:

#!/usr/bin/env bash

Another common bash shebang (force bash):

#!/bin/bash

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Common Ifs

Check File Exist:

if [ -e <file_name> ]; then

Check Variable Valid:

if [ -v Variable ]; then

Check variable not zero length:

if [ -z "$variable" ]; then

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Syntax Checking

Debugging (Print everything):

bash -x <your script>

or

set -x (activate debug) / set +x (deactivate debug)

Syntax Checking:

bash -n <filename>

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Useful Tools

Compare two files, show only lines unique to file 1:

comm -2 -3 <file 1> <file 2>

Show only first few bytes of a file:

head -c6     #only first 6 bytes

Do some simple math:

expr 9 / 3

Easy awk (here selecting first item with delimiter ' '):

cut -d " " -f 1

Capitalizing characters (translate characters)

tr "a-z" "A-Z"

Use xargs to remove unnecessary heading and trailing space:

echo "   google  " | xargs     # result = "google"

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Parameters Expansion:

Check https://wiki.bash-hackers.org/syntax/pe for details.