Bash Cheatsheet

This article is a bit old and the content may be outdated, so please refer to it with caution and remember to check the latest official materials (such as documentation, etc.)

`**/*` not recursive? #

shopt -s globstar

shell

Recursive `chmod` #

From https://stackoverflow.com/a/11512211/8810271

Want set all files with mode 644 and all sub directories 755?

find /opt/lampp/htdocs -type d -exec chmod 755 {} \;
find /opt/lampp/htdocs -type f -exec chmod 644 {} \;

bash

chmod 644 {} \; specifies the command that will be executed by find for each file. {} is replaced by the file path, and the semicolon denotes the end of the command (escaped, otherwise it would be interpreted by the shell instead of find).

If Statement #

if [ $1 -gt 100 ]
then
    echo Hey that\'s a large number.
fi

bash

Single Square Brackets #

These are frequently used, and search CONDITIONAL in bash man page for more.

Operator	Description
! EXPRESSION	The EXPRESSION is false.
-n STRING	The length of STRING is greater than zero.
-z STRING	The lengh of STRING is zero (ie it is empty).
STRING1 = STRING2	STRING1 is equal to STRING2
STRING1 != STRING2	STRING1 is not equal to STRING2
INTEGER1 -eq INTEGER2	INTEGER1 is numerically equal to INTEGER2
INTEGER1 -gt INTEGER2	INTEGER1 is numerically greater than INTEGER2
INTEGER1 -lt INTEGER2	INTEGER1 is numerically less than INTEGER2
-d FILE	FILE exists and is a directory.
-e FILE	FILE exists.
-r FILE	FILE exists and the read permission is granted.
-s FILE	FILE exists and it's size is greater than zero (ie. it is not empty).
-w FILE	FILE exists and the write permission is granted.
-x FILE	FILE exists and the execute permission is granted.

From https://stackoverflow.com/a/31366734/8810271
[ is just a regular command with a weird name.
] is just an argument of [ that prevents further arguments from being used.
Nothing is altered in the way that Bash parses the command. In particular, < is redirection, && and || concatenate multiple commands, ( ) generates subshells unless escaped by \, and word expansion happens as usual.
[[ X ]] is a single construct that makes X be parsed magically. <, &&, || and () are treated specially, and word splitting rules are different. There are also further differences like = and =~.
[ is a built-in command (compgen -b), and [[ is a keyword (compgen -k)

That's why you need to type spaces after [ and before ]. Furthermore, there is no need to type an entire if statement to debug a condition. Just type [[ 0 > 1 ]] and see your shell prompt (if configured) !

Listing awfully named files in order #

Sometimes, you get files like photo_1.jpg, photo_2.jpg, ..., photo_10.jpg, ...

If using plain *.jpg, you will get photo_1.jpg, photo_10.jpg, ..., photo_2.jpg, ...

Quite annoying, isn't it? Solve it by ls -v, namely sort by version.

From man page:

Sort by WORD instead of name: none (-U), size (-S), time (-t), version (-v), extension (-X)