From 5e7d94a3e6d8b9ab9a91758afad3bff68fe645e0 Mon Sep 17 00:00:00 2001 From: Jan Danielzick Date: Sat, 24 Sep 2022 10:40:36 +0000 Subject: [PATCH] Update 'posix_shell.md' --- posix_shell.md | 193 ++++++++++++++++++++++++++++++++++++------------- 1 file changed, 142 insertions(+), 51 deletions(-) diff --git a/posix_shell.md b/posix_shell.md index e87fc3e..05a2c00 100644 --- a/posix_shell.md +++ b/posix_shell.md @@ -1,11 +1,13 @@ # Shell Cheatsheet -I am only going over POSIX shell things here as there are a bunch -of extensions found in some shells / on some systems but not others. +## Introduction + +I am only going over POSIX shell things here as there are a bunch of extensions +found in some shells / on some systems but not others. The sections below should apply to any UNIX-like system. -This cheatsheet will by no means be a comprehensive guide to UNIX -(it’s a cheatsheet, not a handbook) but you can always find out -more information about any given command right there on the shell. +This cheatsheet is by no means a comprehensive guide to UNIX shell scripting +(it’s a cheatsheet, not a handbook, and I don’t know everything) but you can usually +find out more information about any given command right there on the shell. Many systems come with manuals, try `man COMMAND` or `info COMMAND` to find out more about most utilities @@ -14,6 +16,7 @@ Alternatively, if no manual is available, passing the flag `-h` or `--help` will usually print some information. I find myself consulting these constantly because nobody is expected to remember everything beyond the basics and it’s usually faster than googling it. +That said, googling for a solution is generally also a viable option. If you are trying to learn how to use the shell, I recommend you start with the examples section at the end and work your way backwards from there @@ -22,7 +25,7 @@ I also highly recommend you check the manuals or help information for any command you want to use - especially if it was given to you by a stranger on the internet. -## Shell features: +## Shell Features The shell’s main tasks are providing a text based user interface to an operating system as well as running scripts. @@ -42,14 +45,14 @@ Relative paths are just the equivalent of an absolute path with the working dire In many cases, just the name of a file or subdirectory inside the working directory is used. You can change to another working directory using `cd` (change directory) like so: `cd PATH_TO_DIRECTORY`. -When no path is specified, `cd` will take you to the current user’s home directory (usually...). +When no path is specified, `cd` takes you to the current user’s home directory (usually...). **Streams** Normally, there are two output streams and one input stream for a running program: Standard output (stdout), standard error (stderr), and standard input (stdin). By default, they are passed to the stdout, stderr, and stdin of the shell, -which will end up in your terminal when you run a shell interactively. +which end up in your terminal when you run a shell interactively. There are two notable exceptions to this which you can specify: **Redirects**: You can send the output of a command to a file instead of @@ -82,58 +85,146 @@ They can be added to the environment of subsequent commands using `export VARIAB To alter the environment for just one command, prepend it with the variable like so: `VARIABLE=VALUE COMMAND`. Variables can be unset using `unset VARIABLE`. -exit code -if, esle -while, for -pipes, redirects -ctrl-c +Unknown or unset variables are ignored. -*Builtin commands:* -- `exit` exit (optionally with an exit code though that feature is more useful for scripts) -- `jobs` show programs that are currently running in the background -- `fg` get a program from the background in the foreground -wait +**Control Flow** -__Basic commands:__ -- `sh` -- `export` -- `unset` -- `cd` [change directory] (see "Working Directory" section) -- `pwd` [print working directory] self-explanatory -- `ls` [list] Get a listing of the specified directory. If none given, the current directory is used. -- `cat` [concatenate] Originally created to concatenate files, it is most commonly used to print the contents of a file to the standard output. It takes file paths as arguments. -- `mv` [move] Move or rename a file or directory. It takes two arguments: Origin and destination. -- `cp` [copy] Copy a file or directoy. It takes two arguments: Origin and destination. -- `touch` +When a command finishes running, it produces an exit code that gives some rudimentary status information +Usually, an exit code of 0 indicates success and anything else an error of some kind. + +This can be used in control structures (while loops, conditions) or directly accessed using the special +shell variable `$?`. + +If statements are built as follows: +```sh +if COMMAND; then + OTHER + COMMANDS + HERE +else + MORE + COMMANDS + HERE +fi +``` +There are also two operators that can be used as short forms of if and else: `&&` and `||`. +They can be combined and chained as needed. +Commands that are chained using `&&` only run if the previous command exited with exit code 0. +Commands that are chained using `||` run if the previous command *didn’t* exit with exit code 0. +In many cases, this is used as follows: `COMMAND && THEN_COMMAND || ELSE_COMMAND` + + +While loops are built as follows: +```sh +while COMMAND; do + OTHER + COMMANDS + HERE +done +``` +The `[ ... ]` seen in many if statements and while loops is not actually part of the shell syntax +but an alias to the `test` command (see below). + +For loops are different in that they act as "for each" instead of checking a condition. +```sh +for VARNAME in STRING; do + COMMANDS + USING $VARNAME + HERE +done +``` +You can break out of a loop using `break`. + +You can end the shell session or script using `exit`. It optionally takes an exit code as argument. + +**Job Control** + +You can send a process to the background by adding an `&` at the end of the command. +The `jobs` command shows programs that are currently running in the background. +You can get a process from the background into the foreground using `fg`. +Alternatively, if you just want to wait for all jobs to finish, `wait` does that. + +**Miscellaneous** + +Spaces separate arguments and newlines separate commands. To override this, use quoted strings. +Single-quoted strings are taken verbatim, variables in double-quoted strings get replaced. + +The POSIX shell is case-sensitive but it’s not whitespace-sensitive meaning you can put +multiple whitespace characters and they count as one or get ignored. +You can put multiple commands on the same line using semicolons. +This is commonly used for formatting as it makes scripts more readable. + +The built-in `read` command can be used in scripts to read input into a variable. + +Command line arguments given to a script end up in the variables `$0` through `$9`. +They can be moved to the left using `shift`. `$0` starts out being the name of the script itself. + +You can send Ctrl+c to the terminal to send an interrupt to the process running inside it. +Note that this is a feature of the terminal, not of the shell itself. +Usually, this is used to stop whatever child process of the shell is running in the foreground. + +## Commands + +This section gives you a list of commonly used UNIX utilities. +This does not include shell-builtins as I have covered them above already. +Some shells have built-in versions of some of these commands but they should mostly work +the same as their dedicated counterparts. + +- `sh` (shell) The shell itself, can also be used to invoke scripts - `echo` output text to standard output -- `test` [more commonly known as `[ ... ]`] used to check whether a condition is true or false, result is passed back using exit code -- `df` [disk free] show available disk space -- `du` [disk usage] show how much space a given file or directoy is using -- `su` [switch user] self-explanatory -- `rm` [remove] remove a file or directoy -- `mkdir` [make directory] self-explanatory -- `find` find a file or directoy by specified criteria +- `pwd` (print working directory) +- `ls` (list) Get a listing of the specified directory. If none given, the current directory is used. +- `cat` (concatenate) Originally created to concatenate files, it is most commonly used to print + the contents of a file to the standard output. +- `mkdir` (make directory) self-explanatory +- `touch` Change the access and modification timestamp of a file. Also commonly used to create empty files. +- `mv` (move) files or directories +- `cp` (copy) files or directories +- `rm` (remove) files or directories +- `test` (more commonly known as `[ ... ]`) used to check conditions, result is passed back using exit code +- `true` returns exit code 0 +- `false` returns non-zero exit code - `grep` search for regular expressions in text -- `ps` -- `id` -- `uname` [UNIX name] get information about the operating system - usually name, architecture, and build information -- `chmod` [change mode] change file permissions -- `chown` [change owner] change file ownership -- `sed` [stream editor] edit text on the fly -- `vi` unholy abommination of a text editor :) +- `tr` (text replace) replace all occurrences of a character in text +- `sed` (stream editor) edit text on the fly +- `uname` (UNIX name) get information about the operating system +- `id` (identity) get information about users +- `su` (switch user) +- `ps` (process status) get information about running processes - `kill` kill a given process -- `less` or `more` print only a screen worth of text at a time and wait so you have the time to read it +- `find` find a file or directoy by specified criteria +- `chmod` (change mode) change file permissions +- `chown` (change owner) change file ownership +- `df` (disk free) show available disk space +- `du` (disk usage) show how much space a given file or directoy is using +- `vi` unholy abommination of a text editor :) +- `more` print only a screen worth of text at a time and wait so you have the time to read it -__Commonly used special files__ -. .. -dev null -dev urandom +## Commonly Used Special Files + +- `.` The working directory +- `..` Parent directory of the working directory +- `/dev/null` Your personal trash can to redirect all the output you don’t care about to. +Also known as a black hole or the indefinite void. :) +- `/dev/urandom` Source of infinite random data +- `/dev/zero` Source of infinite null bytes + +## Honorable Mentions -__Honorable mentions:__ The following things won’t be available on all systems, but are really handy when they are: -- `free` shows information about used/free RAM on Linux systems + +- `less` better version of `more` - `nano` nice user-friendly terminal text editor available on many systems - `htop` nice user-friendly terminal task manager - `which` easy way to find out where the binary for a given command is located +- `sudo` become root (or any other user) by authenticating the current user +instead of the target user given the current user is permitted to do so - useful when root login is disabled +- `free` shows information about used/free RAM on Linux systems +- `/dev/stdin`, `/dev/stdout`, and `/dev/stderr` special files for you guessed what - `sl` a great way to infuriate anyone who happens to type too quickly -- `sudo` become root (or any other user) by authenticating the current user instead of the target user given the current user is permitted to do so - useful when root login is disabled \ No newline at end of file + +# Examples + +Below are some examples that you can use to better understand above material. + +... to be finished another time ... \ No newline at end of file