Shell script resources

BashResources.md

Useful commands

pushd / popd

• Change the current directory/folder and store the previous folder/path for use by the popd command.

Example

[user@server /usr/ports] $ pushd /etc
/etc /usr/ports
[user@server /etc] $ popd
/usr/ports
[user@server /usr/ports] $

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dirname $0

• Quick way to retrieve the path of the bash script that is being executed.

Example 1

#!/bin/bash
DIRECTORY=$(cd `dirname $0` && pwd)
echo $DIRECTORY

Example 2

#!/bin/bash
 
echo $0
 
full_path=$(realpath $0)
echo $full_path
 
dir_path=$(dirname $full_path)
echo $dir_path
 
examples=$(dirname $dir_path )
echo $examples
 
data_dir="$examples/data"
echo "DATA: $data_dir"

Output

./examples/shell/relative.sh
/home/gabor/work/code-maven.com/examples/shell/relative.sh
/home/gabor/work/code-maven.com/examples/shell
/home/gabor/work/code-maven.com/examples
DATA: /home/gabor/work/code-maven.com/examples/data

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$#

• tells the number of input arguments the script was passed.

Example 1

if [ $# -eq 0 ]
  then
    echo "No arguments supplied"
fi

Example 2

if [ -z "$1" ]
  then
    echo "No argument supplied"
fi

• In this example, it is not safe to use "$1" check, if it is not sure there is always an additional argument.
• Better reference the argument, if we are sure if such argument exists. In all other cases, check the count of arguments using $# and if it is greater than zero, reference the other arguments.

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command

• POSIX compatible way of checking if a command exists or not.

Example 1

if ! command -v COMMAND &> /dev/null
then
    echo "COMMAND could not be found"
    exit
fi

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history

• command to retrieve all typed commands in bash terminal

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which

• which looks for an executable file by searching for it in the directories in the PATH environmental variable.

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strace

• The strace utility is very powerful to learn what a new or running process is doing supporting a variety of monitoring options.
• Very useful tool in troubleshooting https://linux-audit.com/the-ultimate-strace-cheat-sheet/

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shell

• in GNU makefile
• $(shell command)
• The shell function accepts a single argument that is expanded (like all arguments) and passed to a subshell for execution. The standard output of the command is then read and returned as the value of the function. Sequences of newlines in the output are collapsed to a single space. Any trailing newline is deleted. The standard error is not returned, nor is any program exit status.

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find

• count files recursively using find
• find -type f | wc -l

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