Tutorial: A bit of Bash

--D. Thiebaut (talk) 11:03, 26 November 2013 (EST)

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A quick review of some useful Bash commands that can be used on the command line, or that can be included in bash scripts

Creating a bash script

• first line should contain name of bash shell

#! /bin/bash

• make the script executable

chmod +x scriptname

Variables

• Declaration

file="hello.txt"
file=`ls -1 *.txt | head -1`     # get the first .txt file in the directory

• In expressions: put a $ in front of it!

URL="http://cs.smith.edu/dftwiki/images/"
file="toto.png"
pathFile=$URL$file
file="alpha"
pathFile=$URL${file}.png      # add braces if string needs to be concatenated to variable

Script Parameters

• assume doit.sh is a script and it is called as follows:

 doit.sh 8 2000 50

• To get the arguments inside the script:

NP=$1    # will get 8
N=$2     # will get 2000
M=$3     # will get 50

• To test if the number of parameters is 3 inside the script:

if [ "$#" -ne 3 ]; then
    echo "syntax: doit.sh NP N M"
    exit
fi

Displaying strings and various quantities

Getting the length of a string variable

name="toto.txt"
echo "length of $name = ${#name}"      #will output 8

Getting extension and name (without extension) of a file

fullPath="/usr/local/bin/toto.txt"
fileName="${fullPath##*/}"
echo "file name with extension = $fileName"
extension="${fullPath##*.}"
fileName="${fileName%.*}"
echo "fullPath = $fullPath"
echo "fileName without extension = $fileName"
echo "extension = $extension"