Bash-Beginners-Guide

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Bash Guide for Beginners 1.2.2.3.3. Interactive shell behavior Differences in interactive mode: • Bash reads startup files. • Job control enabled by default. • Prompts are set, PS2 is enabled for multi-line commands, it is usually set to ">". This is also the prompt you get when the shell thinks you entered an unfinished command, for instance when you forget quotes, command structures that cannot be left out, etc. • Commands are by default read from the command line using readline. • Bash interprets the shell option ignoreeof instead of exiting immediately upon receiving EOF (End Of File). • Command history and history expansion are enabled by default. History is saved in the file pointed to by HISTFILE when the shell exits. By default, HISTFILE points to ~/.bash_history. • Alias expansion is enabled. • In the absence of traps, the SIGTERM signal is ignored. • In the absence of traps, SIGINT is caught and handled. Thus, typing Ctrl+C, for example, will not quit your interactive shell. • Sending SIGHUP signals to all jobs on exit is configured with the huponexit option. • Commands are executed upon read. • Bash checks for mail periodically. • Bash can be configured to exit when it encounters unreferenced variables. In interactive mode this behavior is disabled. • When shell built-in commands encounter redirection errors, this will not cause the shell to exit. • Special built-ins returning errors when used in POSIX mode don't cause the shell to exit. The built-in commands are listed in Section 1.3.2. • Failure of exec will not exit the shell. • Parser syntax errors don't cause the shell to exit. • Simple spell check for the arguments to the cd built-in is enabled by default. • Automatic exit after the length of time specified in the TMOUT variable has passed, is enabled. More information: • Section 3.2 • Section 3.6 • See Chapter 12 for more about signals. • Section 3.4 discusses the various expansions performed upon entering a command. 1.2.2.4. Conditionals Conditional expressions are used by the [[ compound command and by the test and [ built-in commands. Expressions may be unary or binary. Unary expressions are often used to examine the status of a file. You only need one object, for instance a file, to do the operation on. There are string operators and numeric comparison operators as well; these are binary operators, requiring two objects to do the operation on. If the FILE argument to one of the primaries is in the form /dev/fd/N, then file descriptor N is checked. If the FILE argument to one of the primaries is one of /dev/stdin, /dev/stdout or /dev/stderr, then file descriptor 0, 1 or 2 respectively is checked. Conditionals are discussed in detail in Chapter 7. Chapter 1. Bash and Bash scripts 10
More information about the file descriptors in Section 8.2.3. 1.2.2.5. Shell arithmetic The shell allows arithmetic expressions to be evaluated, as one of the shell expansions or by the let built-in. Evaluation is done in fixed-width integers with no check for overflow, though division by 0 is trapped and flagged as an error. The operators and their precedence and associativity are the same as in the C language, see Chapter 3. 1.2.2.6. Aliases Aliases allow a string to be substituted for a word when it is used as the first word of a simple command. The shell maintains a list of aliases that may be set and unset with the alias and unalias commands. Bash always reads at least one complete line of input before executing any of the commands on that line. Aliases are expanded when a command is read, not when it is executed. Therefore, an alias definition appearing on the same line as another command does not take effect until the next line of input is read. The commands following the alias definition on that line are not affected by the new alias. Aliases are expanded when a function definition is read, not when the function is executed, because a function definition is itself a compound command. As a consequence, aliases defined in a function are not available until after that function is executed. We will discuss aliases in detail in Section 3.5. 1.2.2.7. Arrays Bash provides one-dimensional array variables. Any variable may be used as an array; the declare built-in will explicitly declare an array. There is no maximum limit on the size of an array, nor any requirement that members be indexed or assigned contiguously. Arrays are zero-based. See Chapter 10. 1.2.2.8. Directory stack The directory stack is a list of recently-visited directories. The pushd built-in adds directories to the stack as it changes the current directory, and the popd built-in removes specified directories from the stack and changes the current directory to the directory removed. Content can be displayed issuing the dirs command or by checking the content of the DIRSTACK variable. More information about the workings of this mechanism can be found in the Bash info pages. 1.2.2.9. The prompt Bash Guide for Beginners Bash makes playing with the prompt even more fun. See the section Controlling the Prompt in the Bash info pages. Chapter 1. Bash and Bash scripts 11
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cathy ~> ls -ld [[:digit:]]* drwxrw
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Bash Guide for Beginners p Print te
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sandy ~> sed -n '/a text/,/This/p'
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Bash Guide for Beginners # This is
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Chapter 6. The GNU awk programming
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Bash Guide for Beginners 121script.
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Bash Guide for Beginners amd.conf a
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Bash Guide for Beginners record1 re
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Bash Guide for Beginners 6.3.6. The
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Chapter 7. Conditional statements I
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Bash Guide for Beginners Table 7-2.
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Bash Guide for Beginners 7.1.2.2. N
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[root@elegance root]# We switch to
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Bash Guide for Beginners echo "Prop
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Bash Guide for Beginners We use the
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Bash Guide for Beginners case EXPRE
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Upon successful condition testing,
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Bash Guide for Beginners bit better
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8.2. Catching user input 8.2.1. Usi
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Bash Guide for Beginners michel ~/t
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Bash Guide for Beginners # DO NOT E
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File descriptor 5 Bash Guide for Be
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Bash Guide for Beginners The result
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Bash Guide for Beginners ♦ Compre
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Bash Guide for Beginners for i in `
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touch pic-`date +%s`.jpg sleep 300
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DISKFUL=$(df -h $WEBDIR | grep -v F
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;; 2) $FORTUNE startrek ;; 3) $FORT
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[carol@octarine ~/test] ls -a ./ ..
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Bash Guide for Beginners 9.7.2. Exa
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Chapter 10. More on variables In th
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Bash Guide for Beginners [bob in ~]
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if [ -z $1 ]; then echo "Usage: $0
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10.3.3. Transformations of variable
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Bash Guide for Beginners The opposi
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Chapter 11. Functions In this chapt
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The commands after && are only exec
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zip2 "$TAR" echo "...done." >> "$LO
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Chapter 12. Catching signals In thi
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Bash Guide for Beginners maud: ~> k
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Bash Guide for Beginners 2. Write a
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Bash Guide for Beginners jobs kill
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if [ $i -eq 5 ] ArrowUp+Enter or !!
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cd Change directory. cdp/cdplay An
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Bash Guide for Beginners gimp grep
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Send and receive mail. man Read man
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Bash Guide for Beginners rsh RPM Pa
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W Bash Guide for Beginners w Show w
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Index A aliases Section 3.5.1 ANSI-
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Bash Guide for Beginners Section 3.
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J Bash Guide for Beginners K kill k
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eadonly Section 10.1.3 regular expr
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Bash Guide for Beginners vi(m) Sect
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