`inpath`

Posted on In

Coding along with the book Wicked Cool Shell Scripts- 101 Scripts for Linux, OS X, and UNIX Systems.

This is the script from Page 11:

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#!/bin/bash

in_path()
{
        cmd=$1
        ourpath=$2      
        result=1
        oldIFS=$IFS     
        IFS=":"
 
        for directory in $ourpath; do

        # If i double quoted `$ourpath`, it will not word split, and the loop will only run once.

                if [ -x "$directory/$cmd" ]; then
                        result=0
                fi
        done
 
        IFS=$oldIFS
        return $result
}

checkForCmdInPath()
{
        var=$1
 
        if [ ! "$var" = "" ]; then
                if [ "${var:0:1}" = "/" ]; then
                	# Checks whether the command represented by the `$var` is an absolute path or not. 
                        if [ ! -x "$var" ]; then
                                return 1
                        fi
                elif ! in_path "$var" "$PATH"; then
                        return 2
                fi
        fi
}

Positional parameters

See here

A Quick Word

It is good practice to put double quotes around variables in Bash because it prevents word splitting and globbing.

  • Word splitting is the process of breaking up a string into separate words based on whitespace. If a variable contains spaces, without quotes, Bash will treat each space-separated word as a separate argument. This can cause unexpected behavior if the variable is used as an argument to a command.

  • Globbing is the process of expanding wildcard characters such as * and ? into a list of matching filenames. If a variable contains a glob character, without quotes, Bash will try to expand it into a list of matching filenames. This can also cause unexpected behavior if the variable is used as an argument to a command.

By putting double quotes around variables, Bash treats the variable as a single argument, preserving any whitespace or glob characters within the variable. This helps to ensure that the variable is interpreted correctly and the script behaves as expected.

Special variables

$IFS

The internal field separator (IFS) is a special variable in Bash that specifies the delimiter used to separate fields in a string. By default, the IFS is set to whitespace (i.e., space, tab, and newline characters).

`$?

Display the return values of functions.

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echo $?

You can use it immediately after calling a function with return value.

Slice

[ "${var:0:1}" = "/" ];

The slicing starts at index 0 and lasts for the duration of a single character.

Compare

String comparison

= tests if two strings are equal. For example, if [ "$string1" = "$string2" ] would be true if $string1 and $string2 have the same value.

Pattern matching

== tests if a string matches a pattern using globbing, which is a way to match filenames based on wildcard characters. For example, if [[ "$string" == a* ]] would be true if $string starts with the letter “a”.