2.6 Command Quick Reference Page

Type: Reference page, no required sequential reading

Scope: Bash/Zsh common interactive operations; examples prioritized for GNU/Linux validation, PowerShell syntax differs

Identity and Command Source

Target	Command	Note
Current user	whoami	Output varies with the account
Current Shell process	ps -p $$ -o comm=	$SHELL is typically the login Shell
Command type and all candidates	type -a name	Can view aliases, functions, and built-in commands
First callable command	command -v name	Usually more portable than which
Bash built-in help	help cd	Applies only to Bash built-in commands

Navigation

Goal	Command
Current directory	pwd
List entries	ls
Include hidden entries and details	ls -la
Enter a directory	cd path
Parent directory	cd ..
Previous directory	cd -
Home directory	cd

~ typically expands to the home directory, and . is the current directory. Quote path variables: cd "$dir".

Files and Directories

Goal	Command	Note
Temporary directory	workdir=$(mktemp -d)	Record and clean up after use
Create directory	mkdir -p a/b	-p also creates missing parent directories
Create empty file/update time	touch file	Existing content is not cleared
Write exactly one line	printf '%s\n' text > file	> will overwrite
Append a line	printf '%s\n' text >> file	>> appends
Copy a file	cp -- src dst	-- ends option parsing
Move or rename	mv -- old new	May overwrite existing targets, depending on options
Delete a file	rm -- file	Typically does not go to the trash
Delete an empty directory	rmdir dir	Refuses if the directory is not empty
Recursive copy	cp -R -- src dst	Metadata retention depends on options and platform

Environment and PATH

Goal	Command
Current shell variable	NAME=value
Export to new sub-processes	export NAME
Print a variable literally	printf '%s\n' "$NAME"
Temporarily add tool directory	PATH="$dir:$PATH"; export PATH
Delete variable	unset NAME

The startup file varies depending on Bash/Zsh, login/interactive mode, and the system. Do not blindly overwrite or append ~/.bashrc, ~/.profile, and ~/.zshrc.

Wildcards and Quoting

Syntax	Meaning
*.log	Any .log name
file-?.txt	? matches one character
file-[13].txt	Matches 1 or 3
"$path"	Parameters remain a single parameter after expansion
-- "$path"	Prevent paths starting with - from being treated as options

The behavior of unmatched wildcards varies depending on the shell and options. Preview the targets before a batch operation. Changing a file extension does not convert its format.

Standard Streams, Pipes, and Redirection

Goal	Syntax
Redirect standard output	command >out.log
Append standard output	command >>out.log
Save standard error separately	command 2>err.log
Merge output and error	command >all.log 2>&1
Connect two programs	`left
See failures in any part of a pipeline	set -o pipefail

Redirection occurs from left to right, and 2>&1 >out.log is not equivalent to >out.log 2>&1. Pipes default to connecting standard output.

One Minute Before Deletion

pwd
ls -ld -- "$target"
find "$target" -print

Set $target to the path you intend to inspect before using this read-only snippet. Before deletion, confirm that the variable is nonempty, the path is correct, and the scope is limited. Do not execute dangerous commands to test whether the system will protect you.

When Details Are Required

• GNU Bash: Shell Expansions explains the order of brace expansion, variables, commands, filenames, and process substitution.
• GNU Bash: Redirections provides the complete rules for file descriptors and the left-to-right processing of redirections.
• GNU Findutils: Safe File Name Handling explains why -print0 can preserve special filenames.
• Linux pgrep(1) lists the options implemented by procps; macOS uses its own man pages to check for differences.

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