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Shell Reference

Introductory Guides

The shell is a powerful tool, and with great power comes great responsibility. The following warnings are not intended to frighten, but to give a sense of respect for the power of shell commands.

Most commands are perfectly safe, and often when they do something unexpected it can be fixed with some work. We will do our best to warn you of commands with greater potential for destruction, but no documentation is perfect.

We are not responsible for accidental deletions or overwrites caused inadvertently, or otherwise, by any commands run by researchers. Be warned that directories, files and file contents that are deleted or overwritten cannot be restored by us under any circumstances. Researchers are responsible for maintaining backups of their files. If in doubt about a command please contact Contact Us for guidance.

Educational Resources

The internet has thousands of guides for using the shell. Rather than devise our own complete lesson plan for using the shell, we recommend finding and using one of the high-quality lessons available on the internet. The Software Carpentries group offers a number of high-quality online lesson plans for introductory computing and data science tools. While these lesson plans are intended to be delivered by an instructor in a classroom setting, they can be still be useful to follow solo. For the shell lesson, see https://swcarpentry.github.io/shell-novice/.

At the shell prompt, you can also use the command curl cheat.sh/<command> to get a simple-to-understand explanation of what the command does and how to use it (see curl). Below is an example for the pwd command.

curl of site cheat.sh/pwd

Reference

Command Concepts

Commands are entered at the prompt. The prompt can take many forms, typically something like one of the following. Common features are: (1) a prompt character, often the dollar sign $; (2) a caret to indicate where characters will be inserted when you type, typically a blinking underscore _ or rectangle; (3) color to enhance meaning of various components.

  • Bash on Cheaha...

    bash prompt on cheaha

  • Git Bash on Windows Desktop...

    bash prompt on windows desktop

  • Oh My Zsh on Debian...

    oh my zsh on debian

Commands take the form command [optional] <required>. The word command should be replaced with the literal name of the command, such as pwd, ls and cd, among many others.

The text [optional] is for flags and inputs that are not required to run the command or that have default values. These flags can be useful for modifying the behavior or output of the command.

The text [required] is for flags and inputs that are required to run the command. These must be supplied by the user or the command will not function or produce an error.

Flags start with the character - as with the -l flag in ls -l (see ls). Flags that do not require input can be combined as ls -al. Flags that require input may not be combined as with the flags -n and -m 2 in grep -n -m 2 pattern textfile.txt (see grep).

All inputs are separated by the space character Space. If you wish to or must use a space character in an input, that input must be surrounded by quotation marks. Note that single quotes and double quotes have different behavior. Single quotes '' interpret all characters between them literally. Double quotes "" interprets special characters. In most cases, especially with variable contents, double quotes "" are preferred.

All commands are run in a process. By default, commands run at the shell prompt are run in the shell process, and wait for execution to stop before returning control to you. It is possible to regain control earlier in a number of ways.

Warning

Copying commands from rich-text sources, such as .pdf, Microsoft Office and webpages, can result in copying special or invisible unicode characters. These characters can cause commands to behave unexpectedly and can be difficult to diagnose. Instead, please try pasting your command into a plain-text editor, like notepad, before copying to the shell prompt.

How do I regain control of the prompt while a command is running?

  • Running commands may be terminated using Ctrl+C. Pressing it once will request a graceful termination of the running command. Pressing it more than one will attempt to immediately kill the program.
  • Open a new shell terminal and use that instead.
  • Start the command as command [optional] <required> &. Note the trailing ampersand character &, which causes the command to be run asynchronously in the background.

How do I terminate a process running asynchronously?

Danger

The commands listed here can cause loss of work by termination of incorrect processes if not used carefully.

To kill a process running in another shell terminal or running in the background, use either kill or pkill together with an appropriate signal flag. The flag -15 sends SIGTERM which will allow the program to terminate itself gracefully. The flag -9 sends SIGKILL and will immediately terminate the process, in case -15 is not working.

  • kill <signal> <pid> if you know the process id <pid> of the process. Use ps -u <username> to see your running processes. On Cheaha you can use ps -u $USER as a shortcut.
  • pkill <signal> <name-pattern> if you know the name of the process.

Warning

Using pkill requires carefully thinking of an appropriate name pattern. An incorrect name pattern can cause unwanted termination of processes that may be important to you. Process termination cannot be stopped or undone.

Special (Escaped) Characters

Backslash \ is used to write literal versions of certain special characters. Backslash is also called the escape character, and the special characters are also called escape sequences. Character escape sequences are useful in situations where you need a representation of a character, instead of the result of pressing the corresponding key on your keyboard. For example, if you want to store a newline character in a string then you can't just press Enter. If you did, you would immediately execute the command before you finished the string. Instead you can type \n, which is the escape sequence for a newline character.

  • \t is interpreted as a tab character. The plain-text equivalent of pressing Tab.
  • \n is interpreted as a newline character. The plain-text equivalent of pressing Enter.
  • \\ is interpreted as a single backslash.

At the shell prompt, double quotes "" interpret newline characters, while single quotes '' do not.

Below is an example file containing escaped characters (shown in nano), and its interpreted output (shown with echo).

example file with escaped characters

example of interpreted escaped characters

Piping and Command Chains

Commands may be composed into chains using pipes with the pipe character |. For example, ls -l | wc -l counts the number of lines returned by ls -l (see ls and wc). Warning This construct does not accurately count the number of items in a directory, and is only for demonstration purposes. Do not use this exact command chain in practice.

example of piping ls into wc

Redirects

Command inputs and outputs may be redirected with the characters < for input and > for output. Output redirects using > overwrite the contents of existing files and are destructive. Using >> in place of > appends contents to a file, rather than overwriting the contents.

  • Inputs come from STDIN or 0.
  • Typical output is written to STDOUT or 1 and errors are written to STDERR or 2.
  • ls -l 1> dirlist stores the directory listing to the file dirlist. In this case using > is the same as 1>.

    example of redirecting stdout to a file

  • ls -l doesnotexist 1> error 2>&1 stores the error message to the file error. The text 2>&1 means write STDERR to STDOUT.

    example of redirecting stderr to a file

  • wc -l 0< lines reads the contents of file lines and counts the number of lines. Note that for wc this is not needed, but may be needed for other commands. In this case using < is the same as 0<.

    example of redirecting a file to stdin

For more information on the commands used in the examples, see ls, wc and cat.

Danger

Output redirects using > are destructive. The contents of the target file are immediately overwritten when the command is executed. It is not possible to recover the contents of the file under any circumstances. Researchers are responsible for maintaining backups of their files.

Path Concepts

  • The working directory is the directory you are currently in and may be identified using the command pwd.
  • Dot . is a shortcut for the working directory. This is only used in some contexts.
  • Double dot .. is a shortcut for the immediate parent directory of whatever comes before it.
  • Twiddle ~ is a shortcut for your home directory.
  • Forwardslash / is the path to the root directory of the filesystem, which has no parent.
  • Files and directory names starting with . are hidden.
  • Paths are formed of text-based directory names separated by /

    • Absolute paths start at the root directory, e.g. /home/user/documents/.
    • Relative paths start at the working directory, e.g. bin.

    Below are examples of constructed paths tested with ls.

    examples of paths

Glob Syntax

  • Glob is a shorthand syntax for dealing with many files and directories matching simple patterns.

    • Question mark ? matches a single character. c?t matches both cat and cut.
    • Star * matches any string. c* matches cat, cut, and clatter. *.png matches all png files.
    • Double star ** matches any number of directories with any names. **/*.png matches all png files within any subdirectory of the working directory. This is not commonly used, but extremely useful for some applications.

    Below are examples of glob usage tested with ls.

    examples of globs

Environment Concepts

  • Environment variables may be assigned by using var=value where var is the variable name and value is its value. Below is an example tested with echo.

    example of variable assignment and checking

  • Environment variable values may be expanded by using "${var}" where var is the variable name.

    • On Cheaha...
      • "${HOME}" expands to the path to your home directory.
      • "${USER_DATA}" expands to the path to your data/user/<username> directory.
      • "${USER}" expands to your user name.
    • Always use double quotes around variables. DO use "${HOME}", do NOT use ${HOME}. Double quotes ensure that space characters in expanded values are handled appropriately. Below, note the error occurring without double quotes due to the spaces in the directory name. Double quoting the variable fixes the error. The commands used to test are ls, cd and pwd.

      example of double quote use

    • Expanding a variable that isn't defined returns an empty string and does not produce an error, but may cause unexpected behavior.

  • Environment variables may be expanded in paths and command arguments to save effort and time.

    • "${USER_DATA}/project/inputs"
    • command ${VARIABLE}

Warning

Modifying, changing or overwriting existing environment variables while in a shell session can result in unexpected behavior. The environment can be reset to its default starting state by exiting the shell session and starting a new session.

Script Concepts

  • Scripts are a way to bundle many commands together and execute them in sequence.
  • Scripts should start with the intended interpreter using a hash-bang like #!/bin/bash.
    • Most commonly bash is the intended interpreter on our systems.
    • Other shell interpreters may be installed and used, but are not necessary.
  • To execute the script given by the hash-bang or she-bang (pronounced shih-bang) #!, use ./script.sh in the folder containing the script. Executable permissions must be set to use a script this way, with chmod u+x <script-path>.
  • To execute the script using a specific interpreter use bash <script-path>, or replace bash with your preferred interpreter. Beware that not all interpreters behave the same way. Executable permissions do not need to be set to use a script this way.
  • Space-separated arguments may be passed to a script when executed in the same way as any other command.

Script Arguments

Arguments or parameters are passed to a command or script as a space separated list. Arguments may be referred to using numeric variables. The following list contains examples of variable references to arguments.

  • "${0}" is the execution path. If you use ./script.sh then "${0}" will be ./script.sh.
  • "${1}", "${2}", etc., are the first, second, etc., space separated variables. Calling ./script.sh hello world will have $1 = hello and $2 = world. Many arguments may be passed this way.
  • ${@} is all arguments except "${0}". Important!! Note that double quotes " are not used! Double quoting would bind all the arguments together. If you need to pass a group of arguments to another script, be cautious about using quotes.
  • ${@: 2} is all arguments starting with "${2}".
  • ${@: 2:2} is the second and third argument.
  • ${@: -2} is the last two arguments.
  • ${@: -2:1} is the second to last argument only.

Below is an example script file, hash-bang not shown, demonstrating how each argument variable works, and its interpreted output. The text editor nano is used to display the file and chmod is used to modify file permissions.

example file showing numeric argument variables

example of interpreted numeric argument variables

Tip

Using the shell requires some defensive techniques. Never use the space character Space in variables, directory names, file names, etc. Instead, only use letters, numbers and the underscore character _. In bash it is also allowed to use the hyphen character -, but this may not be portable to other shell interpreters. Names should only start with one or more letters or numbers.

Do protect yourself from others who might use the space character Space by always double quoting your variables like "${var}" instead of ${var} or $var.

Commands for Solutions to Common Problems

Below is a reference guide to various commands through the lens of problems to be solved.

Note

When you see words surrounded by angle brackets like <name>, you should not take that as a literal part of the command. In the case of <name> you would replace it with whatever name is appropriate.

Important

If you are using Cheaha and working with more than a few files or directories, or the files are large, please run your shell commands in a Job Context.

Danger

It is safest to assume that any command run at the shell cannot be undone. Be especially aware of the rm command, which is destructive. We do not maintain backups of any files, so once those files are removed or deleted they cannot be recovered by us under any circumstances. Researchers are responsible for maintaining backups of their files.

Show working directory (pwd)

Use pwd, which stands for present working directory.

example usage of pwd

List files and directories (ls)

Below are common uses of ls, short for "list", used to display directory contents and examine details of files and directories. It may be used to check file permissions when using chmod.

  • Visible files only, list: ls. Multiple entries per line.

    example usage of ls

  • Visible files only, table: ls -l. One entry per line and shows permissions, size in bytes on disk, and timestamp.

    example usage of ls -l

  • Visible and hidden files, list ls -a. Same as ls, but has hidden files and directories and . and ... Example below is truncated to conserve page space.

    example usage of ls -a

  • Visible and hidden files, table ls -al. Same as ls -l but has hidden files and directories and . and ... Example below is truncated to conserve page space.

    example usage of ls -al

Examine disk usage (du)

Use du to examine disk usage of files and directories. By default all values are given in bytes, use the flag -h to give values in K, M, G and T for kilobytes, megabytes, gigabytes and terabytes, respectively. Use the flag -s to summarize space used by directories. Below is an example of du -sh. Note that only directories with read permissions can be examined by du.

When culling files to conserve storage space, it helps to find the largest files and directories. To find the ten largest, use the command du -sh .[^.]* * | sort -hr | head -n10 in the top-level directory of your data. To better understand this command chain, see also sort, head and piping.

Change working directory (cd)

  • To change to a different directory, use cd <directory-path>.
    • The variable <path> may be relative, like my/path. This will move to the subdirectory my/path within the working directory. Relative directory paths may contain multiple .. shortcuts to indicate parent directories.
    • Or <path> may be absolute, like /my/path which will move to the directory /my/path, starting at the root of the filesystem. Recall the root directory is just /.
  • To move up to the parent of the working directory, use cd ...

Below are examples of cd usage, tested with ls.

example usage of cd

Copy files and directories (cp)

Below are use cases with associated commands, and examples tested using ls.

  • Single file, change name: cp -a <source-file-path> <destination-file-path>.

    example of cp on a single file

  • Single file, keep name: cp -a <source-file-path> <destination-directory-path>.

  • Directory: cp -a <source-directory-path> <destination-directory-path>.

    example of cp on a directory

  • Glob: cp -a <source-path-glob> <destination-directory-path>.

Move files and directories mv

Below are use cases with associated commands, and examples tested using ls.

  • Single file, change name: mv <source-file-path> <destination-file-path>.

    example of mv on a single file

  • Single file, keep name: mv <source-file-path> <destination-directory-path>.

  • Directory: mv <source-directory-path> <destination-directory-path>.

    example of mv on a directory

  • Glob: mv <source-path-glob> <destination-directory-path>.

Delete files and directories (rm, rmdir)

Danger

The rm command is destructive and cannot be undone. We do not maintain backups of any files, so files that are removed or deleted cannot be recovered by us under any circumstances. Researchers are responsible for maintaining backups of their files.

Below are use cases with associated commands, and examples tested using ls.

  • Single file: rm <file-path>.

    example of rm on a single file

  • Empty Directory: rmdir <directory-path>.

  • Directory with Contents: rm -r <directory-path>.

    example of rm on a directory

  • Glob: rm <file-path-glob>.

Warning

Careless use of the directory and glob forms of rm can lead to unwanted data loss. Be sure to double check your commands before executing.

Download files from internet sources (curl)

Use curl to download files and webpages from internet sources. By default curl writes to STDOUT. If you wish to save the output to a file, use the -o <file-path> command. Note that curl does not transform, encode or decode the data in any way, and it is saved exactly as received from the supplied url.

By far the most common usage is to download a file. To do so use curl -o <file-path> <url>, where <file-path> is the desired local path to save and <url> is the web address of the source data.

Create directories (mkdir)

Use mkdir <directory-name>. Example below is tested using ls.

example of mkdir

Create files (touch)

Use touch <file-path>. Example below is tested using ls.

example of touch

Edit plain-text files (nano)

Use nano <file-path>. If the file exists, it will be opened. If the file does not exist, it will be stored in memory until saved. If the contents are not saved they will be discarded and cannot be recovered. You may also use just nano, without a file path, to create a new empty file.

example of nano interface

The character ^ is the Ctrl key, so ^y would require pressing Ctrl+Y. To save use Ctrl+X to open the exit prompt.

example of exit prompt

Press Y for yes to bring up the save prompt.

example of save prompt

If you opened nano using an existing file, or passed in a file path, then the File Name to Write prompt will be autofilled with the given file name. If not, you will need to enter it here. When you have the file name entered as desired, press Enter to save. Below is a demonstration of the shell with the new file listed using ls. Example below is tested using ls.

ls result of new file saved in nano

Searching for text in nano

Use the key combination Ctrl+W to search for text. Follow the prompts as they appear to navigate.

Count lines, words and characters (wc)

  • Lines: wc -l <file-path>.
  • Words: wc -w <file-path>.
  • Characters: wc -m <file-path>.
  • All: wc <file-path>.

Below are example outputs of a file named newfile containing the text hello world followed by a blank line. Note the file is redirected to wc from STDIN.

example wc output

Display file contents (cat, less)

Use cat <file-path> to display the contents of the file at <file-path>.

example of cat

If the contents are too long to be displayed on one screen, you can pipe the output to less to add scrolling functionality by using cat <file-path> | less. Use the keyboard key Q to stop using less.

Below are two images showing what less looks like. Note the last line of each image, which indicates you are using the less environment. The first image is an example where there is more text below the visible text. The second image is an example at the bottom of the text.

example of cat less with more text below

example of cat less at the bottom of the text

Examine start and end of file (head, tail)

To display only the first two lines use head -n 2 <file-path>. Use tail instead of head for the last two lines.

Below is an image showing the use of head and tail on a file with four lines. The file was created by redirecting the echo command to a file, using the special newline character to add line breaks.

example of head and tail

Sort file contents (sort)

  • Alphabetical: sort <file-path>.
  • Numeric: sort -n <file-path>.
  • Ignore case: sort -i <file-path>.

Lines in the input file are returned in sorted order. The results are displayed in stdout. To write the sorted result to a file use sort <file-path> > <new-file-path>.

Below is an image showing the use of head and tail on a file with four lines. The file was created by redirecting the echo command to a file, using the special newline character to add line breaks.

file used with sort example

example of sort

Test a command (echo)

Use echo "<command>" to see the expanded command without executing it. For example

echo "cp -a $USER_DATA/mydir $HOME"
# prints
cp -a /data/user/<username>/mydir /home/<username>

Use the -e flag if you need to interpret escaped characters such as \t for tab or \n for new line. The command being examined in the example below is cp

example of echo used for command test

Search for text (grep)

Use grep "<pattern>" "<file-path>" to search for <pattern> in the file at <file-path>. Use the -n flag to display line numbers with results.

Below is an example of grep -n on a file. The number at the start of the result line is the line number of the pattern match. The matched portion is shown in red while other text on the same line is shown in white. The file was created by redirecting the echo command to a file, using the special newline character to add line breaks. We are looking for the literal text "echo" within the file.

example of grep showing line number

Close the session (exit)

Use exit.

Clear the shell display (clear)

Use clear.

Where is a command located? (which)

Use which <command-name>. The command being searched for is ls.

example of which

What does a command do? (whatis, man)

For builtin command and aliases there are two distinct options for learning more. The command being examined is ls.

  • Use whatis <command-name> to get a brief summary of the command.
  • Use man <command-name> to get the full help file in a less environment. Use q on your keyboard to exit less.

example of whatis

Remotely access shell on other machines (ssh)

See our SSH Section for more detailed information.

Remotely access or transfer files between machines (sftp)

See our Remote Access - Data Transfer Section for more detailed information.

Submit and manage jobs on Cheaha

See our SLURM Section for more detailed information.

Manage permissions of files and directores (chmod)

Use chmod with the least permissions needed to accomplish a task.

Permission management is an important part of managing access and control of files and directories.

Danger

Please carefully consider security when working in shared spaces like Cheaha. Setting private directories or files as readable by other users can inadvertently expose sensitive or protected information and may violate IT policy, FERPA, HIPAA or some combination.

There are legitimate use cases for truly shared spaces. Please Contact Us if you need to share information with other users or collaborators and aren't sure of how to do so securely.

What Permissions Do

Setting the permissions of a file affect the contents of only that file. A read-only file can still be deleted by users with write permissions in its parent directory.

  • Read permissions allow viewing and copying contents of a file.
  • Write permissions allow changing the contents of a file, including deleting all of the contents.
  • Execute permissions allow using the file as an executable. Helpful for scripts and compiled programs. Scripts and interpreted language files, like Python, must also have read permission set.

Setting the permissions of directories affects what can be done with contained files and directories.

  • Read permission allows the use of ls within the directory. cp may be used to copy files from the directory to somewhere else.
  • Write permission allows creation of files and directories, as well as the use of touch, mv and rm on files and directories, within the directory. cp may be used to copy files into the directory.
  • Executable permission allows setting the directory as working directory and the use of cd into the directory.

Permissions are not inherited from their parent directory.

How to Check Permissions

Use ls -ald <path> to see the permissions on the file or directory at <path>. The -d flag lists directories instead of their contents.

Patterns for Setting Permissions

Two separate patterns can be used to set or change permissions on files and directories. Either may be used, but they cannot be combined in a single use of chmod. In the example images below, the command ls is used to check permissions and the command cat is used to display the contents of the script.

  1. Symbols

    The letter and symbol pattern is in the form a=r. There are three parts.

    1. A collection of letters denoting who, e.g. a in this case. Multiple letters may be used.

      • u the owner of the file or directory
      • g the owner's group members
      • o users outside the owner's group
      • a all users (same as ugo).
    2. A symbol indicating how to change the permissions

      • = set permissions
      • - remove permissions
      • + add permissions
    3. A collection of letters denoting which permissions to change. Multiple letters may be used.

      • r read
      • w write (change the contents)
      • x execute

    To add executable permission for only the owner chmod u+x <file-path>. Useful for custom scripts and compiled executables you will use directly from the command line. To set read-only permission for everyone use chmod a=r <file-path>.

    Note

    Using = to set permissions will both add and remove permissions. Using a=r will take away existing write and execute permissions.

    Below is an example of chmod used symbolically to set user execute permissions on a script. Note the error before permissions are set.

    example of symbolic chmod

  2. Numerals

    Bit mask patterns are in the form 755. Each digit is the sum of three binary bits. The bits are

    • 4 read
    • 2 write
    • 1 execute

    The left digit is the owner's permissions. Middle digit is the owner's group. Right digit is users outside the owner's group.

    Setting chmod 755 means the following:

    • For the owner, set 4 read, 2 write and 1 execute. 4+2+1=7.
    • For the owner's group, set 4 read, 1 execute. 4+1=5.
    • For other users, set 4 read, 1 execute. 4+1=5.

    Setting 755 is a common pattern for system-wide scripts. Because the 4 read and 1 executable bits are set for all users, it can be called from anywhere by any other script, and not just the owner. However, the 2 write bit is set only for the owner, so other users cannot modify the contents of the script.

    Below is an example of chmod used numerically to add execution permissions to all users. Note the error before permissions are set.

    example of numeric chmod

Examples

  • chmod u+x script.sh adds execute permission for you, to a script.
  • chmod 755 script.sh makes a script readable and executable by all, but only writeable by you. This is a common permission for non-sensitive files and directories.
  • chmod 740 sensitive_directory make a directory readable by you and your group, and writeable and executable by only you. Other users cannot delete files in this folder.
  • chmod ug=r notes.txt followed by chmod o-rwx notes.txt makes a file read-only for you and your group and removes all permissions for other users.

Manage researcher access to files and directories (getfacl, setfacl)

Construction

Under construction.


Last update: August 16, 2022