Bash - shell scripting examples

While this site is primarily about Perl programming, a real Perl Maven needs to be familiar with a lot of other technologies. Not only Perl. This page is a collection of expressions in Bash, one of the most commonly used Unix/Linux shell language.

I am not a frequent Shell-script writer, but for example I needed this for the build script of DWIM Perl for Linux.

sh-bang

#!/bin/bash -e

See Writing robust shell scripts for an explanation about the -e.

if statements

Does this directory exist?

TEST_DIR=/opt/myperl
if [ -d "$TEST_DIR" ]; then
    echo $TEST_DIR already exists. Exiting!
    exit
fi

Does file exist?

    PERL_SOURCE_ZIP_FILE=perl-5.18.1.tar.gz
    if [ ! -f "$PERL_SOURCE_ZIP_FILE" ]; then
        wget http://www.cpan.org/src/5.0/$PERL_SOURCE_ZIP_FILE
    fi

Is variable empty?

X=abc
echo "$X"
if [ "$X" == "" ]; then
    echo yes
fi

or maybe even better to use the -z operator:

X=abc
echo "$X"
if [ -z "$X" ]; then
    echo yes
fi

Command line arguments

if [ $# == 0 ]; then
    echo "empty"
else
    for x in "$@"; do
        echo "$x"
    done
fi

(Remember to put " around the $@ or it will incorrectly handle command-line parameters with spaces in them, like "Perl Maven")

See other comparison operators.

for loop

for tool in gcc make cmake; do
    echo "Checking $tool"
done

Conditionally install packages on CentOS

for tool in gcc make cmake; do
    echo "Checking $tool"
    $tool --version || {
        echo "Installing $tool"
        yes | yum install $tool || { echo "Could not install $tool"; exit 1; }
    }
done

Assign values to a variable from external code

BUILD_HOME=$(pwd)

Avoid backticks!

$() does the same is better readable and can be stacked.

cwd=$(pwd)

$( $(which test) -d $DIRECTORY)

[[

It was also recommended to use [[ ]] instead of [ ], but I am not yet sure why.

Quoting

Also always double quote your variables, always (unless you have a good reason not to - which requires you to know the reason. if you don't, quote your variables.) And using lowercase or camel case variable names avoids accidental overriding of environment variables.

Thanks

the article was updated based on the comments of Edward Macke, Dirk Deimeke, Norbert Varzariu, Adam Outler