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Shell Script Basics Writing a shell script is like riding a bike. You fall off and scrape your knees a lot at first. With a bit more experience, you become comfortable riding them around town, but also quickly discover why most people drive cars for longer trips. Shell scripting is generally considered to be a glue language, ideal for creating small pieces of code that connect other tools together. While shell scripts can be used for more complex tasks, they are usually not the best choice. If you have ever successfully trued a bicycle wheel (or paid someone else to do so), that’s similar to learning the basics of shell scripting.
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If you don’t true your scripts, they wobble. Put another way, it is often easy to write a script, but it can be more challenging to write a script that consistently works well. This chapter and the next two chapters introduce the basic concepts of shell scripting.
The remaining chapters in this document provide additional breadth and depth. This document is not intended to be a complete reference on writing shell scripts, nor could it be. It does, however, provide a good starting point for beginners first learning this black art. Shell Script Dialects There are many different dialects of shell scripts, each with their own quirks, and some with their own syntax entirely.
Because of these differences, the road to good shell scripting can be fraught with peril, leading to script failures, misbehavior, and even outright data loss. To that end, the first lesson you must learn before writing a shell script is that there are two fundamentally different sets of shell script syntax: the Bourne shell syntax and the C shell syntax. The C shell syntax is more comfortable to many C programmers because the syntax is somewhat similar. However, the Bourne shell syntax is significantly more flexible and thus more widely used. For this reason, this document only covers the Bourne shell syntax. The second hard lesson you will invariably learn is that each dialect of Bourne shell syntax differs slightly.
This document includes only pure Bourne shell syntax and a few BASH-specific extensions. Where BASH-specific syntax is used, it is clearly noted. The terminology and subtle syntactic differences can be confusing—even a bit overwhelming at times; had Dorothy in The Wizard of Oz been a programmer, you might have heard her exclaim, 'BASH and ZSH and CSH, Oh My!' Fortunately, once you get the basics, things generally fall into place as long as you avoid using shell-specific features. Stay on the narrow road and your code will be portable.
Some common shells are listed below, grouped by script syntax: Bourne-compatible shells. sh.
bash. zsh. ksh C-shell-compatible shells. csh. tcsh. bcsh (C shell to Bourne shell translator/emulator) Many of these shells have more than one variation. Most of these variations are denoted by prefixing the name of an existing shell with additional letters that are short for whatever differentiates them from the original shell.
For example:. The shell pdksh is a variant of ksh. Being a public domain rewrite of AT&T's ksh, it stands for 'Public Domain Korn SHell.' (This is a bit of a misnomer, as a few bits are under a BSD-like open source license. However, the name remains.). The shell tcsh is an extension of csh.
It stands for the TENEX C SHell, as some of its enhancements were inspired by the TENEX operating system. The shell bash is an extension of sh. It stands for the Bourne Again SHell. (Oddly enough, it is not a variation of ash, the Almquist SHell, though both are Bourne shell variants. This should not be confused with the dash shell—an ash-derived shell used in some Linux distributions—whose name stands for the Debian Almquist SHell.) And so on. In general, with the exception of csh and tcsh, it is usually safe to assume that any modern login shell is compatible with Bourne shell syntax.
Note: Because the C shell syntax is not well suited to scripting beyond a very basic level, this document does not cover C shell variants in depth. For more information, see. She Sells C Shells The C shell is popular among some users as a shell for interacting with the computer because it allows simple scripts to be written more easily. However, the C shell scripting language is limited in a number of ways, many of which are hard to work around. For this reason, use of the C shell scripting language for writing complex scripts is not recommended.
For more information, read “CSH Programming Considered Harmful” at. Although many of the language flaws it describes are fixed by some modern C shells, if you are writing a script that must work on multiple computers across different operating systems, you cannot always guarantee that the installed C shell will support those extensions. However, the C shell scripting language has its uses, particularly for writing scripts that set up environment variables for interactive shell environments, execute a handful of commands in order, or perform other relatively lightweight chores. To support such uses, the C shell syntax is presented alongside the Bourne shell syntax within this 'basics” chapter where possible.
Outside of this chapter, this document does not generally cover the C shell syntax. If after reading this, you still want to write a more complex script using the C shell programming language, you can find more information in on the C shell in the manual page for csh. Shell Variables and Printing What follows is a very basic shell script that prints “Hello, world!” to the screen.
#!/bin/sh echo 'Hello, world!' The first thing you should notice is that the script starts with ‘ #!’. This is known as an interpreter line.
If you don’t specify an interpreter line, the default is usually the Bourne shell ( /bin/sh). However, it is best to specify this line anyway for consistency.
The second thing you should notice is the echo command. The echo command is nearly universal in shell scripting as a means for printing something to the user’s screen.
(Technically speaking, echo is generally a shell builtin, but it also exists as as standalone command, /bin/echo. You can read more about the difference between the builtin version and the standalone version in and.) If you’d like, you can try this script by saving those lines in a text file (say “helloworld.sh”) in your home directory. Then, in Terminal, type. #!/bin/sh FIRSTARGUMENT='$1' echo 'Hello, world $FIRSTARGUMENT!' Type or paste this script into the text editor of your choice (see for help creating a text file) and save the file in your home directory in a file called test.sh.
Once you have saved the file in your home directory, type ‘ chmod a+x test.sh’ in Terminal to make it executable. Finally, run it with ‘./test.sh leaders’.
You should see “Hello, world leaders!” printed to your screen. This script provides an example of a variable assignment. The variable $1 contains the first argument passed to the shell script.
In this example, the script makes a copy and stores it into a variable called FIRSTARGUMENT, then prints that variable. You should immediately notice that variables may or may not begin with a dollar sign, depending on how you are using them. If you want to dereference a variable, you precede it with a dollar sign.
The shell then inserts the contents of the variable at that point in the script. For all other uses, you do not precede it with a dollar sign.
='myfirstcommandlineargument' This is clearly not what you want (and produces an error). Because of the order in which the statement is evaluated, the above assignment statement would still fail with an error even if FIRSTARGUMENT were nonempty.
(If you really want to assign a value to a variable whose name is in a different variable, use eval, as described in.) You should also notice that the argument to echo is surrounded by double quotation marks. This is explained further in the next section,. #!/bin/sh FIRSTARGUMENT='$1' echo 'Hello, world $FIRSTARGUMENT!' Notice that the echo statement is followed by a string surrounded by quotation marks. Normally, the shell uses spaces to separate arguments to commands. Outside of quotation marks, the shell would treat “Hello,” and “world” as separate arguments to echo. By surrounding the string with double quote marks, the shell treats the entire string as a single argument to echo even though it contains spaces.
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To see how this works, save the script above as test.sh (if you haven’t already), then type the following commands./test.sh leaders and citizens./test.sh 'leaders and citizens' The first line above prints “Hello, world leaders!” because the space after “leaders” ends the first argument ( $1). Inside the script, the variable $1 contains “leaders”, $2 contains “and”, and $3 contains “citizens”. The second line above prints “Hello, world leaders and citizens!” because the quotation marks on the command line cause everything within them to be grouped as a single argument. Notice also that there are similar quotation marks on the right side of the assignment statement. Mkdir '/tmp/My Folder' FILENAME='/tmp/My Folder' ls '$FILENAME' ls $FILENAME The above example creates a directory in /tmp called “My Folder”.
(Don’t worry about deleting it because /tmp gets wiped every time you reboot.) It then attempts to list the files in that directory. The first time, it uses quotation marks. The second time, it does not.
Notice that the shell misinterprets the command the second time as being an attempt to list the files in /tmp/My and the files in Folder. Handling Quotation Marks in Strings In modern Bourne shells, expansion of variables, occurs after the statement itself is fully parsed by the shell.
(See in for more information.) Thus, as long as the variable is enclosed in double quote marks, you do not get any execution errors even if the variable’s value contains double-quote marks. However, if you are using double quote marks within a literal string, you must quote that string properly./test.sh ' 'leaders ' prints the phrase “Hello, world “leaders”!” The details of quotes as they apply to variable expansion are explained in. (Variable safety with shells that predate this behavior is generally impractical.
Fortunately, the modern behavior has been the norm since the mid-1990s.) Shell scripts also allow the use of single quote marks. Variables between single quotes are not replaced by their contents. Be sure to use double quotes unless you are intentionally trying to display the actual name of the variable. You can also use single quotes as a way to avoid the shell interpreting the contents of the string in any way.
These differences are described further in. Exporting Shell Variables One key feature of shell scripts is that variables are typically limited in their scope to the currently running script. The scoping of variables is described in more detail in. For now, though, it suffices to say that variables generally do not get passed on to scripts or tools that they execute. Normally, this is what you want.
Most variables in a shell script do not have any meaning to the tools that they execute, and thus represent clutter and the potential for variable namespace collisions if they are exported. Occasionally, however, you will find it necessary to make a variable's value available to an outside tool.
To do this, you must export the variable. These exported variables are commonly known as environment variables because they affect the execution of every script or tool that runs but are not part of those scripts or tools themselves. A classic example of an environment variable that is significant to scripts and tools is the PATH variable. This variable specifies a list of locations that the shell searches when executing programs by name (without specifying a complete path).
For example, when you type ls on the command line, the shell searches in the locations specified in PATH (in the order specified) until it finds an executable called ls (or runs out of locations, whichever comes first). The details of exporting shell variables differ considerably between the Bourne shell and the C shell. Thus, the following sections explain these details in a shell-specific fashion. Using the export Builtin (Bourne Shell) Generally speaking, the first time you assign a value to an environment variable such as the PATH variable, the Bourne shell creates a new, local copy of this shell variable that is specific to your script.
Any tool executed from your script is passed the original value of PATH inherited from whatever script, tool, or shell that launched it. With the BASH shell, however, any variable inherited from the environment is automatically exported by the shell. Thus, in some versions of OS X, if you modify inherited environment variables (such as PATH) in a script, your local changes will be seen automatically by any tool or script that your script executes. Thus, in these versions of OS X, you do not have to explicitly use the export statement when modifying the PATH variable. Because different Bourne shell variants handle these external environment variables differently (even among different versions of OS X), this creates two minor portability problems:.
A script written without the export statement may work on some versions of OS X, but will fail on others. You can solve this portability problem by using the export builtin, as described in this section. A shell script that changes variables such as PATH will alter the behavior of any script that it executes, which may or may not be desirable. You can solve this problem by overriding the PATH environment variable when you execute each individual tool, as described in. To guarantee that your modifications to a shell variable are passed to any script or tool that your shell script calls, you must use the export builtin. You do not have to use this command every time you change the value; the variable remains exported until the shell script exits. Export PATH='/usr/local/bin:$PATH' # or PATH='/usr/local/bin:$PATH' export PATH Either of these statements has the same effect—specifically, they export the local notion of the PATH environment variable to any command that your script executes from now on.
There is a small catch, however. You cannot later undo this export to restore the original global declaration. Thus, if you need to retain the original value, you must store it somewhere yourself. In the following example, the script stores the original value of the PATH environment variable, exports an altered version, executes a command, and restores the old version. ORIGPATH='$PATH' PATH='/usr/local/bin:$PATH' export PATH # Execute some command here-perhaps a # modified ls command. Ls PATH='$ORIGPATH' If you need to find out whether an environment variable (whether inherited by your script or explicitly set with the export directive) was set to empty or was never set in the first place, you can use the printenv command to obtain a complete list of defined variables and use grep to see if it is in the list. (You should note that although printenv is a csh builtin, it is also a standalone command in /usr/bin.) For example.
DEFINED=`printenv grep -c '^VARIABLE='` The resulting variable will contain 1 if the variable is defined in the environment or 0 if it is not. Overriding Environment Variables for Child Processes (Bourne Shell) Because the BASH Bourne shell variant automatically exports all variables inherited from its environment, any changes you make to preexisting environment variables such as PATH are automatically inherited by any tool or script that your script executes. (This is not true for other Bourne shell variants; see for further explanation.) While automatic export is usually convenient, you may sometimes wish to change a preexisting environment variable without modifying the environment of any script or tool that your script executes. For example, if your script executes a number of tools in /usr/local/bin, it may be convenient to change the value of PATH to include /usr/local/bin. However, you may not want child processes to also look in /usr/local/bin. This problem is easily solved by overriding the environment variable PATH on a per-execution basis. Consider the following script.
Chmod a+x printmyvar.sh # makes the script executable MYVAR=7./printmyvar.sh # runs the script echo 'MYVAR IS $MYVAR' # prints the variable Notice that the assignment statement MYVAR=7 applies only to the command that follows it. The value of MYVAR is altered in the environment of the command./printmyvar.sh, so the script prints the number 7. However, the original (empty) value is restored after executing that command, so the echo statement afterwards prints an empty string for the value of MYVAR. Thus, to modify the PATH variable locally but execute a command with the original PATH value, you can write a script like this. Setenv VALUE 'Four' unsetenv VALUE set VALUE = 'Four' unset VALUE echo 'VALUE is '$VALUE'.' This will generate an error message. In the C shell, it is not possible to print the value of an undefined variable, so if you think you may need to print the value later, you should set it to an empty string rather than using unset or unsetenv.
If you need to test an environment variable ( not a shell-local variable) that may or may not be part of your environment (a variable set by whatever process called your script), you can use the printenv builtin. This prints the value of a variable if set, but prints nothing if the variable is not set, and thus behaves just like the variable behaves in the Bourne shell. Set DEFINED = `printenv grep -c '^VARIABLE='` The resulting variable will contain 1 if the variable is defined in the environment or 0 if it is not. Overriding Environment Variables for Child Processes (C Shell) Unlike the Bourne shell, the C shell does not provide a built-in syntax for overriding environment variables when executing external commands. However, it is possible to simulate this either by using the env command.
The best and simplest way to do this is with the env command. Env PATH='/usr/local/bin' /bin/ls As an alternative, you can use the set builtin to make a temporary copy of any variable you need to override, change the value, execute the command, and restore the value from the temporary copy. You should notice, however, that whether you use the env command or manually make a copy, the PATH variable is altered prior to searching for the command. Because the PATH variable controls where the shell looks for programs to execute, you must therefore explicitly provide a complete path to the ls command or it will not be found (unless you have a copy in /usr/local/bin, of course).
The PATH environment variable is explained in. As a workaround, you can determine the path of the executable using the which command prior to altering the PATH environment variable. Security Note: If your purpose for overriding an environment variable is to prevent disclosure of sensitive information to a potentially untrusted process, you should be aware that if you use setenv for the copy, the called process has access to that temporary copy just as it had access to the original variable.
To avoid this, be sure to create the temporary copy using the set builtin instead of setenv. Deleting Shell Variables For the most part, in Bourne shell scripts, when you need to get rid of a variable, setting it to an empty string is sufficient. However, in long-running scripts that might encounter memory pressure, it can be marginally useful to delete the variable entirely. To do this, use the unset builtin.