Contents: Bash Reference Sheet

Basic Structures

Compound Commands

Compound commands are statements that can execute several commands but are considered as a sort of command group by Bash.

Command Lists

• { [command list]; }: Execute the list of commands in the current shell.

• ([command list]): Execute the list of commands in a subshell.

Expressions

• (([arithmetic expression])): Evaluates the given expression in an arithmetic context.

  • That means, strings are considered names of integer variables, all operators are considered arithmetic operators (such as ++, ==, >, <=, etc..) You should always use this for performing tests on numbers!

• [[ [test expression] ]]: Evaluates the given expression as a test-compatible expression.

  • All test operators are supported but you can also perform Glob pattern matching and several other more advanced tests. It is good to note that word splitting will not take place on unquoted parameter expansions here. You should always use this for performing tests on strings and filenames!

Loops

• do [command list]; done

  • This constitutes the actual loop that is used by the next few commands.
    The list of commands between the do and done are the commands that will be executed in every iteration of the loop.

• for [name] in [words]

  • The next loop will iterate over each WORD after the in keyword.
    The loop's commands will be executed with the value of the variable denoted by name set to the word.

• for (( [arithmetic expression]; [arithmetic expression]; [arithmetic expression] ))

  • The next loop will run as long as the second arithmetic expression remains true.
    The first arithmetic expression will be ran before the loop starts. The third arithmetic expression will be ran after the last command in each iteration has been executed.

• while [command list]

  • The next loop will be repeated for as long as the last command ran in the command list exits successfully.

• until [command list]

  • The next loop will be repeated for as long as the last command ran in the command list exits unsuccessfully ("fails").

• select [name] in [words]

  • The next loop will repeat forever, letting the user choose between the given words.

    • 
      The iteration's commands are executed with the variable denoted by name's value set to the word chosen by the user. Naturally, you can use break to end this loop.

Builtins

Builtins are commands that perform a certain function that has been compiled into Bash. Understandably, they are also the only types of commands (other than those above) that can modify the Bash shell's environment.

Dummies

• true (or :): These commands do nothing at all.

  • They are NOPs that always return successfully.

• false: The same as above, except that the command always "fails".

  • It returns an exit code of 1 indicating failure.

Declarative

• alias: Sets up a Bash alias, or print the bash alias with the given name.

  • Aliasses replace a word in the beginning of a command by something else. They only work in interactive shells (not scripts).

• declare (or typeset): Assign a value to a variable.

  • Each argument is a new variable assignment. Each argument's part before the equal sign is the name of the variable, and after comes the data of the variable. Options to declare can be used to toggle special variable flags (like read-only/export/integer/array).

• export: Export the given variable to the environment so that child processes inherit it.

  • This is the same as declare -x. Remember that for the child process, the variable is not the same as the one you exported. It just holds the same data. Which means, you can't change the variable data and expect it to change in the parent process, too.

• local: Declare a variable to have a scope limited to the current function.

  • As soon as the function exits, the variable disappears. Assigning to it in a function also doesn't change a global variable with the same name, should one exist. The same options as taken by declare can be passed to local.

• type: Show the type of the command name specified as argument.

  • The type can be either: alias, keyword, function, builtin, or file.

Input

• read: Read a line (unless the -d option is used to change the delimiter from newline to something else) and put it in the variables denoted by the arguments given to read.

  • If more than one variable name is given, split the line up using the characters in IFS as delimiters. If less variable names are given than there are split chunks in the line, the last variable gets all data left unsplit.

Output

• echo: Output each argument given to echo on one line, separated by a single space.

  • The first arguments can be options that toggle special behaviour (like no newline at end/evaluate escape sequences).

• printf: Use the first argument as a format specifier of how to output the other arguments.

  • See help printf.

• pwd: Output the absolute pathname of the current working directory.

  • You can use the -P option to make pwd resolve any symlinks in the pathname.

Execution

• cd: Changes the current directory to the given path.

  • If the path doesn't start with a slash, it is relative to the current directory.

• command: Run the first argument as an application.

  • This tells Bash not to look for an alias, function, builtin or keyword by that name; but skip right ahead to looking through PATH.

• . or source: Makes Bash read the filename given as first argument and execute its contents in the current shell.

  • This is kind of like include in other languages. If more arguments are given than just a filename to source, those arguments are set as the positional parameters during the execution of the sourced code. If the filename to source has no slash in it, PATH is searched for it.

• exec: Run the command given as first argument and replace the current shell with it.

  • Other arguments are passed to the command as its arguments. If no arguments are given to exec but you do specify Redirections on the exec command, the redirections will be applied to the current shell.

• exit: End the execution of the current script.

  • If an argument is given, it is the exit status of the current script (an integer between 0 and 255).

• logout: End the execution of a login shell.

• return: End the execution of the current function.

  • An exit status may be specified just like with the exit builtin.

• ulimit: Modify resource limitations of the current shell's process.

  • These limits are inherited by child processes.

Jobs/Processes

• jobs: List the current shell's active jobs.

• bg: Send the previous job (or job denoted by the given argument) to run in the background.

  • The shell continues to run while the job is running. The shell's input is handled by itself, not the job.

• fg: Send the previous job (or job denoted by the given argument) to run in the foreground.

  • The shell waits for the job to end and the job can receive the input from the shell.

• kill: Send a signal(3) to a process or job.

  • As argument, give the process ID of the process or the jobspec of the job you want to send the signal to.

• trap: Handle a signal(3) sent to the current shell.

  • The code that is in the first argument is executed whenever a signal is received denoted by any of the other arguments to trap.

• suspend: Stops the execution of the current shell until it receives a SIGCONT signal.

  • This is much like what happens when the shell receives a SIGSTOP signal.

• wait: Stops the execution of the current shell until active jobs have finished.

  • In arguments, you can specify which jobs (by jobspec) or processes (by PID) to wait for.

Conditionals And Loops

• break: Break out of the current loop.

  • When more than one loop is active, break out the last one declared. When a number is given as argument to break, break out of number loops, starting with the last one declared.

• continue: Skip the code that is left in the current loop and start a new iteration of that loop.

  • Just like with break, a number may be given to skip out more loops.

Arguments

• set: The set command normally sets various Shell options, but can also set Positional parameters.

  • Shell options are options that can be passed to the shell, such as bash -x or bash -e. set toggles shell options like this: set -x, set +x, set -e, ... Positional parameters are parameters that hold arguments that were passed to the script or shell, such as bash myscript -foo /bar. set assigns positional parameters like this: set -- -foo /bar.

• shift: Moves all positional parameters' values one parameter back.

  • This way, values that were in $1 are discarted, values from $2 go into $1, values from $3 go into $2, and so on. You can specify an argument to shift which is an integer that specifies how many times to repeat this shift.

• getops: Puts an option specified in the arguments in a variable.

  • getopts Uses the first argument as a specification for which options to look for in the arguments. It then takes the first option in the arguments that is mentioned in this option specification (or next option, if getopts has been ran before), and puts this option in the variable denoted by the name in the second argument to getopts. This command is pretty much always used in a loop:

    while getopts abc opt
    do
       case $opt in
          a) ...;;
          b) ...;;
          c) ...;;
       esac
    done

    This way all options in the arguments are parsed and when they are either -a, -b or -c, the respective code in the case statement is executed. Following short style is also valid for specifying multiple options in the arguments that getopts parses: -ac.

Tests

Exit Codes

An Exit Code or Exit Status is an unsigned 8-bit integer returned by a command that indicates how its execution went. It is agreed that an Exit Code of 0 indicates the command was successful at what it was supposed to do. Any other Exit Code indicates that something went wrong. Applications can choose for themselves what number indicates what went wrong; so refer to the manual of the application to find out what the application's Exit Code means.

Testing The Exit Code

• if [command list]; then [command list]; elif [command list]; then [command list]; else [command list]; fi

  • The if command tests whether the last command in the first command list had an exit code of 0.
    If so, it executes the command list that follows the then. If not, the next elif is tried in the same manner. If no elifs are present, the command list following else is executed, unless there is no else statement. To summarize, if executes a list of *command*s. It tests the exit code. On success, the then commands are executed. elif and else parts are optional. The fi part ends the entire if block (don't forget it!).

• while [command list], and until [command list]

  • Execute the next iteration depending on the exit code of the last command in the command list.
    We've discussed these before, but it's worth repeating them in this section, as they actually do the same thing as the if statement; except that they execute a loop for as long as the tested exit code is respectively 0 or non-0.

Patterns

Bash knows two types of patterns. Glob Patterns is the most important, most used and best readable one. Later versions of Bash also support the "trendy" Regular Expressions. However, it is ill-advised to use regular expressions in scripts unless you have absolutely no other choice or the advantages of using them are far greater than when using globs. Generally speaking, if you need a regular expression, you'll be using awk(1), sed(1), or grep(1) instead of Bash.

Glob Syntax

• ?: A question mark matches any character.

  • That is one single character.

• *: A star matches any amount of any characters.

  • That is zero or more of whatever characters.

• [...]: This matches *one of* any of the characters inside the braces.

  • That is one character that is mentioned inside the braces.

    • [abc]: Matches either a, b, or c but not the string abc.

    • [a-c]: The dash tells Bash to use a range.

      • Matches any character between (inclusive) a and c. So this is the same thing as the example just above.

    • [!a-c] or [^a-c]: The ! or ^ in the beginning tells Bash to invert the match.

      • Matches any character that is *not* a, b or c. That means any other letter, but *also* a number, a period, a comma, or any other character you can think of.

    • [[:digit:]]: The [:class:] syntax tells Bash to use a character class.

      • Character classes are groups of characters that are predefined and named for convenience. You can use the following classes:

        alnum, alpha, ascii, blank, cntrl, digit, graph, lower, print, punct, space, upper, word, xdigit

Testing Patterns

• [[ [string] = [glob pattern] ]], or [[ [string] =~ [regular expression] ]]:

  • The [[ command is an improved version of the commonly used [ or test command.
    [ and test are commands you often see in sh scripts to perform tests on files, strings and numbers. [[ can do all these things (but better) and it also provides you with Glob Pattern matching. This syntax causes [[ to return a successful exit code (0) when the given string matches the given glob pattern, or the given regular expression with the second syntax.

• case [string] in [glob pattern]) [command list];; [glob pattern]) [command list];; esac:

  • Using case is handy if you want to test a certain string that could match either of several different glob patterns.
    The command list that follows the *first* glob pattern that matched your string will be executed. You can specify as many glob pattern and command lists combos as you need.

Parameters

Parameters are what Bash uses to store your script data in. There are Special Parameters and Variables.

Any parameters you create will be variables, since special parameters are read-only parameters managed by Bash. It is recommended you use lower-case names for your own parameters so as not to confuse them with the all-uppercase variable names used by Bash internal variables and environment variables. It is also recommended you use clear and transparent names for your variables. Avoid x, i, t, tmp, foo, etc. Instead, use the variable name to describe the kind of data the variable is supposed to hold.

It is also important that you understand the need for quoting. Generally speaking, whenever you use a parameter, you should quote it: echo "The file is in: $filePath". If you don't, bash will tear the contents of your parameter to bits, delete all the whitespace from it, and feed the bits as arguments to the command. Yes, Bash mutilates your parameter expansions by default - it's called Word Splitting - so use quotes to prevent this.
The exception is keywords and assignment. After myvar= and inside [[, case, etc, you don't need the quotes, but they won't do any harm either - so if you're unsure: quote!

Last but not least: Remember that parameters are the data structures of bash. They hold your application data. They should NOT be used to hold your application logic. So while many ill-written scripts out there may use things like GREP=/usr/bin/grep, or command='mplayer -vo x11 -ao alsa', you should NOT do this. The main reason is because you cannot possibly do it completely right and safe and readable/maintainable.
If you want to avoid retyping the same command multiple times, or make a single place to manage the command's command line, use a function instead. Not parameters.

Special Parameters

• 1, 2, ...: Positional Parameters are the arguments that were passed to your script or your function.

  • When your script is started with ./script foo bar, "$1" will become "foo" and "$2" will become "bar". A script ran as ./script "foo bar" hubble will expand "$1" as "foo bar" and "$2" as "hubble".

• *: When expanded, it equals the single string that concatenates all positional parameters using the first character of IFS to separate them (- by default, that's a space).

  • In short, "$*" is the same as "$1x$2x$3x$4x..." where x is the first character of IFS.
    With a default IFS, that will become a simple "$1 $2 $3 $4 ...".

• @: This will expand into multiple arguments: Each positional parameter that is set will be expanded as a single argument.

  • So basically, "$@" is the same as "$1" "$2" "$3" ..., all quoted separately.
    NOTE: You should always use "$@" before "$*", because "$@" preserves the fact that each argument is its separate entity. With "$*", you loose this data! "$*" is really only useful if you want to separate your arguments by something that's not a space; for instance, a comma: (IFS=,; echo "You ran the script with the arguments: $*") -- output all your arguments, separating them by commas.

• #: This parameter expands into a number that represents how many positional parameters are set.

  • A script executed with 5 arguments, will have "$#" expand to 5. This is mostly only useful to test whether any arguments were set: if (( ! $# )); then echo "No arguments were passed." >&2; exit 1; fi

• ?: Expands into the exit code of the previously completed foreground command.

  • We use $? mostly if we want to use the exit code of a command in multiple places; or to test it against many possible values in a case statement.

• -: The dash parameter expands into the option flags that are currently set on the Bash process.

  • See set for an explanation of what option flags are, which exist, and what they mean.

• $: The dollar parameter expands into the Process ID of the Bash process.

  • Handy mostly for creating a PID file for your bash process (echo "$$" > /var/run/foo.pid); so you can easily terminate it from another bash process, for example.

• !: Expands into the Process ID of the most recently backgrounded command.

  • Use this for managing backgrounded commands from your Bash script: foo ./bar & pid=$!; sleep 10; kill "$pid"; wait "$pid"

• _: Expanding the underscore argument gives you the last argument of the last command you executed.

  • This one's used mostly in interactive shells to shorten typing a little: mkdir -p /foo/bar && mv myfile "$_".

Examples: Basic Structures

Compound Commands

Command Lists

• [[ $1 ]] || { echo "You need to specify an argument!" >&2; exit 1; }

  • We use a command group here because the || operator takes just one command.
    We want both the echo and exit commands to run if $1 is empty.

• (IFS=','; echo "The array contains these elements: ${array[*]}")

  • We use parenthesis to trigger a subshell here.
    When we set the IFS variable, it will only change in the subshell and not in our main script. That avoids us having to reset it to it's default after the expansion in the echo statement (which otherwise we would have to do in order to avoid unexpected behaviour later on).

• (cd "$1"; tar -cvjpf archive.tbz2 .)

  • Here we use the subshell to temporarily change the current directory to what's in $1.
    After the tar operation (when the subshell ends), we're back to where we were before the cd command because the current directory of the main script never changed.

Expressions

• ((completion = current * 100 / total))

  • Note that arithmetic context follows completely different parsing rules than normal bash statements.

• [[ $foo = /* ]] && echo "foo contains an absolute pathname."

  • We can use the [[ command to perform all tests that test(1) can do.
    But as shown in the example it can do far more than test(1); such as glob pattern matching, regular expression matching, test grouping, etc.

Loops

• for file in *.mp3; do openssl md5 "$file"; done > mysongs.md5

  • For loops iterate over all arguments after the in keyword.
    One by one, each argument is put in the variable name file and the loop's body is executed.
    
    DO NOT PASS A COMMAND'S OUTPUT TO for BLINDLY!
    for will iterate over the WORDS in the command's output; which is almost NEVER what you really want!

• for (( i = 0; i < 50; i++ )); do printf "%02d," "$i"; done

  • Generates a comma-separated list of numbers zero-padded to two digits.
    (The last character will be a comma, yes, if you really want to get rid of it; you can - but it defeats the simplicity of this example)

• while read _ line; do echo "$line"; done < file

  • This while loop continues so long as the read command is successful.
    (Meaning, so long as lines can be read from the file). The example basically just throws out the first column of data from a file and prints the rest.

• until myserver; do echo "My Server crashed with exit code: $?; restarting it in 2 seconds .."; sleep 2; done

  • This loop restarts myserver each time it exits with a non-successful exit code.
    It assumes that when myserver exits with a non-successful exit code; it crashed and needs to restart; and if it exist with a successful exit code; you ordered it to shut down and it needn't be restarted.

• select fruit in Apple Pear Grape Banana Strawberry; do (( credit -= 2, health += 5 )); echo "You purchased some $fruit.  Enjoy!"; done

  • A simple program which converts credits into health.
    Amazing.

Builtins

Dummies

• while true; do ssh lhunath@lyndir.com; done

  • Reconnect on failure.

Declarative

• alias l='ls -al'

  • Make an alias called l which is replaced by ls -al.
    Handy for quickly viewing a directory's detailed contents.

• declare -i myNumber=5

  • Declare an integer called myNumber initialized to the value 5.

• export AUTOSSH_PORT=0

  • Export a variable on the bash process environment called AUTOSSH_PORT which will be inherited by any process this bash process invokes.

• foo() { local bar=fooBar; echo "Inside foo(), bar is $bar"; }; echo "Setting bar to 'normalBar'"; bar=normalBar; foo; echo "Outside foo(), bar is $bar"

  • An exercise in variable scopes.

• if ! type -P ssh >/dev/null; then echo "Please install OpenSSH." >&2; exit 1; fi

  • Check to see if ssh is available.
    Suggest the user install OpenSSH if it is not, and exit.

Input

• read firstName lastName phoneNumber address

  • Read data from a single line with four fields into the four named variables.

Output

• echo "I really don't like $nick.  He can be such a prick."

  • Output a simple string on standard output.

• printf "I really don't like %s.  He can be such a prick." "$nick"

  • Same thing using printf instead of echo, nicely separating the text from the data.

Execution

• cd ~lhunath

  • Change the current directory to lhunath's home directory.

• command [ "$a" = a ] || echo "$a is not 'a'!"

  • Execute the [ command (not the builtin!) and execute the echo statement if it fails.

• source bashlib; source ~/.foorc

  • Run all the bash code in a file called bashlib which exists somewhere in PATH; then do the same for the file .foorc in the current directory.

• exec 2>/var/log/foo.log

  • Send all output to standard error from now on to a log file.

• echo "Fatal error occurred!  Terminating!"; exit 1

  • Show an error message and exit the script.

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