Moving Beyond Basic Bash SHELL GAMES

LinuxuSer Command Line: Bash Builtins Sebastian Duda, 123RF Duda, Sebastian Moving beyond basic Bash SHELL GAMES Even beginners can benefit from a greater understanding of the Bash has a mixture of single-letter options be- ginning with a hyphen (-) that it inher- shell’s many built-in commands. BY BRUCE BYFIELD ited from earlier incarnations, along with longer options that begin with a double ost GNU/ Linux users know or another shell just like any other com- hyphen (- -) that were added by the GNU that the command line uses mand. In Gnome, you can add options Project. MBash (Bourne Again Shell) [1], by running Gnome Terminal and creat- However, as a command, bash does but what fewer know is that, by default, ing a custom profile. When the profile is behave in some unusual ways. The first they use the plainest version of Bash. created, select Edit | Profiles | Edit | Title argument after the options can be a file The truth is that Bash is a command & Command. Then check the Run a cus- full of commands that Bash runs instead much like any other. It might be the tom command instead of my shell check- of waiting for your input at the key- command that runs other commands – a box (Figure 1). In the Custom command board. Even more significantly, as Bash command interpreter – but, just like any field, enter the form of the Bash com- starts, it refers to /etc/ profile, the generic other command, its behavior can be rad- mand you want to run, then, directly Bash profile, and ~/ .profile or ~./ . ically altered by the options you add below it, select what you want to hap- bash_profile, the customized profile in when starting it up or by running some pen when you exit from the command. the home directory of the current ac- of its built-in commands (builtins). Similarly, in KDE, open Konsole and count, as well as /etc/ bash.bashrc and Moreover, you can even choose an en- create a new profile by selecting Settings ~/ .bashrc for non-login shells. You can tirely different shell through which to in- | Manage Profile. Highlight the new pro- prevent the reading of profile files by teract with your system. file and click Edit Profile | General to adding the --noprofile option and of the Admittedly, many of Bash’s options modify the Bash command (Figure 1). If *bashrc files by adding the --norc option. and builtins [2] are of interest only if you you add the Konsole widget to your Instead, you can force Bash to use a re- are writing scripts. Desktop users are un- panel or folder view, it presents a list of placement for all bashrc files with the likely, for example, to find uses for a profiles from which you can choose to option --rcfile [file]. login shell or builtins like continue or de- suit your needs. Many of Bash’s options are of interest clare. Still, even beginners at the com- mainly to advanced users, although you mand prompt can benefit from learning Options effects might try running --verbose for a while to more about the options available from The basic structure of the Bash com- see which environment variables are in- the command line. mand is no different from any other voked by each command (Figure 2). Modifying Bash is easy. From any command: bash [options] [arguments]. Many, like those that change the startup computer or terminal, you can start Bash And, just like any other command, bash files, modify resources that Bash uses. 88 ISSUE 111 FEBRUARY 2010 Command Line: Bash Builtins LinuxuSer Several builtins give you basic information about your system. For instance, the type command either tells you whether a command is an alias or builtin or it gives the path to the executable (Figure 5). You cannot see system commands like apt-get unless you are logged in as root, but type can satisfy your curiosity about the commands you Figure 1: Whether you run KDE (left) or Gnome (right), you can run a modified version of Bash, or another shell are using and how each is altogether, by making a simple change in your desktop. regarded. For example, cp (copy) is a essential com- However, you might try running the cd command to move to the subdirectory mand and is placed in /bin, whereas command bash --debugger -O extdebug (Figure 4). Another handy mod is cd- sudo, which allows you to perform root to run Bash in debugger mode. Another spell, which tries to correct the misspell- functions from another account, is less moderately advanced option is to start a ings of directory names automatically – essential to your system and is therefore restricted shell with -r, --restricted, or -O although it usually requires the full path placed in /usr/ bin. Also, you can use restricted_shell. A restricted shell is ex- and fails when you use abbreviations type to check to see whether a command actly what it sounds like: one in which like ./ for the current directory. Similarly, is an alias, although you might prefer to some basic actions are not permitted. dirspell corrects misspellings of directory use the alias command instead. These include changing directories, turn- names in file completion. Another builtin that gives you infor- ing off restrictions with set, and another Another mod for file completion is no- mation is jobs, which lists processes and nine or ten actions [3]. caseglob, which ignores the usual dis- whether they are running or stopped – Some system administrators use a re- tinction between upper- and lowercase something that can be hard to tell if you stricted shell to prevent users from mov- letters. Also, checkjobs di- are running ing about in the shell, but this technique rects Bash to display the commands in is a weak security tactic because you do status of running or the background, not need to be much of an expert to start stopped jobs when you especially if a shell that is interactive. Instead, a re- exit Bash. The mailwarn they follow the stricted shell is most valuable when you mod has Bash tell you Figure 3: A restricted shell is just Unix tradition want to test programs that might be when a file containing what it sounds like: a shell in which of not providing buggy or untrusted (Figure 3). By run- mail has been read since some functionality, like changing completion ning such programs in a restricted shell, the last time Bash ac- directories, is not allowed. messages. you can hope to minimize any damage cessed it. Entering the that they cause. bare command will give you a list of the Perhaps the most interesting option is Builtin Commands processes owned by the current user ac- -O [modification], which affects Bash in Many of Bash’s builtins are for scripting, count and the status of each. If you want a number of simple yet productive ways. which is a topic for another day, but in to end a process because it is misbehav- You can start the same modifications addition to shopt, with its modifications ing, you can enter job -l to find its pro- with Bash’s builtin shopt -s [option]. of Bash’s behavior, the builtins also in- cess ID, then enter kill [processID] to Besides the extdebug and restricted_ clude functions that are useful in every- shut it down. shell options already mentioned, some of day desktop computing. Other builtins both give information the most immediately useful modifica- In fact, Bash’s builtins are unavoid- and edit system behavior. For example, tions (mods) affect navigation. For in- able. If you have ever typed pwd to the unadorned command set lists all en- stance, if you run bash -O autocd, Bash check which directory you are in or cre- vironmental variables, whereas other interprets the name of a subdirectory ated an alias so that entering ls is equiv- options allow you to turn off or on stan- that is entered as an alias for using the alent to typing ls --color=auto, then you dard system behavior such as brace ex- have used Bash’s pansion (-B), which is Bash’s ability to builtins. For ex- replace a variable typed in curly braces ample, you can’t with its value. even navigate at Similarly, ulimit -a gives you informa- the prompt with- tion about system resource limitations, Figure 2: If you’ve ever wondered what happens when you run a com‑ out using the cd such as the maximum number of threads mand, run Bash with the verbose option. builtin. allowed or the maximum number of pro- FEBRUARY 2010 ISSUE 111 89 LinuxuSer Command Line: Bash Builtins thing: The owner of a file can zsh described as being to Bash what read, write, or execute it, but Bash is to the Windows command line – neither their group nor any- in other words, vastly superior. That is one else except root can do an exaggeration and, in tcsh’s case, is any of these things. If I probably helped by the fact that tcsh is wanted to allow others to the default shell of FreeBSD. But, with- Figure 4: Here, the ‑O option tells Bash to treat a direc‑ read new files, I could enter out a doubt, these shells possess numer- tory name that is entered as the equivalent of changing umask 4477 to change the ous features that Bash either lacks or has to that directory, and it will try to correct directory default. in less sophisticated form. For instance, names that are misspelled. Yet another builtin worth tcsh not only has a scripting syntax simi- knowing about is history, lar to that of the C programming lan- cesses that a single user can own (Figure which manages the command history guage, but it also has programmable 6).

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