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SH(1) General Commands Manual SH(1) NAME sh, cd, wait, whatis - shell, the standard command programming language SYNOPSIS sh [ -acefiknpstuvx ] [ args ] DESCRIPTION Sh is a command programming language that executes commands read from a terminal or a file. See `Invocation' below for the meaning of argu- ments to the shell. Definitions A blank is a tab or a space. A name is a sequence of letters, digits, or underscores beginning with a letter or underscore. A parameter is a name, a digit, or any of the characters *, @, #, ?, -, $, and !. A word is a sequence of characters and quoted strings set off by opera- tors, blanks, or newlines; see `Quoting'. Commands A simple-command is a sequence of words separated by blanks. The first word specifies the name of the command to be executed. Except as spec- ified below, the remaining words are passed as arguments to the invoked command. The command name is passed as argument 0; see exec(2). The value of a simple-command is its exit status if it terminates normally, or 0200+status if it terminates abnormally; see signal(2) for a list of status values. A pipeline is a sequence of one or more commands separated by |. If there is more than one command, each is run in a subshell; | denotes a pipe(2) connecting the standard output of one command to the standard input of the next. Each command is run as a separate process; the shell waits for the last command to terminate. The exit status of a pipeline is the exit status of the last command. A list is a sequence of one or more pipelines separated by ;, &, &&, or ||, and terminated by ; or &. Of these four symbols, ; and & have equal precedence, which is lower than that of && and ||. The symbols && and || also have equal precedence. A semicolon (;) causes sequen- tial execution of the preceding pipeline; an ampersand (&) causes asyn- chronous execution of the pipeline; the shell does not wait and pro- ceeds as if the pipeline had returned zero exit status. The symbol && (||) causes the list following it to be executed only if the preceding pipeline returns a zero (non-zero) exit status. One or more newlines may follow any sequencing operator (; & && ||). One or more newlines may always be used in place of a single semicolon, and newlines may be freely inserted after any of | ; & && || ;; if do then elif else fi done while until. A command is either a simple-command or one of the following. Unless otherwise stated, the value returned by a command is that of the last simple-command executed in the command. for name [ in word ... ; ] do list ; done A command executes a list of commands once for each word, with name set to each word in turn. If in word ... ; is omitted or replaced by newlines, then the list is executed once for each positional parameter that is set; see `Parameter Substitution'. case word in [ pattern [ | pattern ] ... ) list ;; ] ... esac A command executes the list associated with the first pattern that matches word. The form of the patterns is the same as that used for file-name generation (see `File Name Generation') ex- cept that a slash, a leading dot, or a dot immediately following a slash need not be matched explicitly. Newlines may precede each pattern and replace the last ;; before esac. if list then list [ elif list then list ] ... [ else list ] fi The list following is executed and, if it returns a zero exit status, the list following the first is executed. Otherwise, the lists in clauses are executed in turn until one returns zero status; then the list following the next is executed. Other- wise, the list is executed. If no list or list is executed, then the if command returns a zero exit status. while list do list done A command repeatedly executes the list and, if the exit status of the last command in the list is zero, executes the list; oth- erwise the loop terminates. If no commands in the list are exe- cuted, then the command returns a zero exit status; may be used in place of to negate the loop termination test. (list) Execute list in a sub-shell. {list} list is simply executed. name () command Define a function which is referenced by name. The body of the function is the command. The most useful form of command is a sequence of commands enclosed in braces { }. Execution of func- tions is described under Execution below. These words are only recognized as the first word of a command and when not quoted: if then else elif fi case esac for while until do done. Comments A word beginning with # causes that word and all the following charac- ters up to a newline to be ignored. Command Substitution The standard output from a command enclosed in a pair of grave accents ` ` may be used as part or all of a word; trailing newlines are re- moved. Parameter Substitution The character $ is used to introduce substitutable parameters. There are two types of parameters, positional and keyword. If parameter is a digit, it is a positional parameter. Positional parameters may be as- signed values by set. Keyword parameters (also known as variables) may be assigned values by writing: name=value [ name=value ] ... Pattern-matching is not performed on value. There cannot be a function and a variable with the same name. ${parameter} The value, if any, of the parameter is substituted. The braces are required only when parameter is followed by a letter, digit, or underscore that is not to be interpreted as part of its name. If parameter is * or @, all the positional parameters, starting with $1, are substituted (separated by spaces). Parameter $0 is set from argument zero when the shell is invoked. ${parameter:-word} If parameter is set and is non-null, substitute its value; oth- erwise substitute word. ${parameter:=word} If parameter is not set or is null set it to word; the value of the parameter is substituted. Positional parameters may not be assigned to in this way. ${parameter:?word} If parameter is set and is non-null, substitute its value; oth- erwise, print word and exit from the shell. If word is omitted, the message ``parameter null or not set'' is printed. ${parameter:+word} If parameter is set and is non-null, substitute word; otherwise substitute nothing. In the above, word is not evaluated unless it is to be used as the sub- stituted string, so that, in the following example, pwd is executed only if d is not set or is null: echo ${d:-`pwd`} If the colon (:) is omitted from the above expressions, the shell only checks whether parameter is set or not. The following parameters are automatically set by the shell: # The number of positional parameters in decimal. - Flags supplied to the shell on invocation or by the set command. ? The decimal value returned by the last synchronously exe- cuted command; see exit(2). $ The process number of this shell. ! The process number of the last background command in- voked. The following parameters are used by the shell: HOME The default argument (home directory) for the cd command. PATH The search path for commands; see `Execution'. CDPATH The search path for the cd command. MAIL If this parameter is set to the name of a mail file the shell informs the user of the arrival of mail in the specified file. The file is inspected every three min- utes. HISTORY If this parameter is set to the name of a writable file, the shell appends interactive input to the file, for use by the command =(1). PS1 Primary prompt string, by default PS2 Secondary prompt string, by default IFS Internal field separators, normally space, tab, and new- line. The shell gives default values to PATH, PS1, PS2 and IFS. HOME is set by login(8). Blank Interpretation After parameter and command substitution, the results of substitution are scanned for internal field separator characters (those found in IFS) and split into distinct arguments where such characters are found. Explicit null arguments ("" or '') are retained. Implicit null argu- ments (those resulting from parameters that have no values) are re- moved. File Name Generation Following substitution, each command word is scanned for the characters *, ?, and [. If one of these characters appears the word is regarded as a pattern. The word is replaced with alphabetically sorted file names that match the pattern. If no file name is found that matches the pattern, the word is left unchanged. The directories . and .. (initially or after a /) are only matched by patterns beginning with an explicit period. The character / itself must be matched explicitly. * Matches any string, including the null string. ? Matches any single character. [...] Matches any one of the enclosed characters. A pair of characters separated by - matches any character lexically between the pair, inclusive. If the first character fol- lowing the opening is a any character not enclosed is matched. Quoting These characters have a special meaning to the shell and terminate a word unless quoted: ; & ( ) | < > { } newline space tab (The characters { and } need not be quoted inside a ${} construction.) A character may be quoted (i.e., made to stand for itself) by preceding it with a \. The pair \newline is ignored. All characters enclosed between a pair of single quote marks '' (except a single quote) are quoted. Inside double quote marks "" parameter and command substitu- tion occurs and \ quotes the characters \, `, ", and $. "$*" is equiv- alent to "$1 $2 ...", whereas "$@" is equivalent to "$1" "$2" .... Prompting When used interactively, the shell prompts with the value of PS1 before reading a command. If at any time a newline is typed and further input is needed to complete a command, the secondary prompt (i.e., the value of PS2) is issued. Input/Output Before a command is executed, its input and output may be redirected using a special notation interpreted by the shell. The following may appear anywhere in a simple-command or may precede or follow a command and are not passed on to the invoked command; substitution occurs be- fore word or digit is used: <word Use file word as standard input (file descriptor 0). >word Use file word as standard output (file descriptor 1). If the file does not exist it is created; otherwise, it is truncated to zero length. >>word Use file word as standard output. If the file exists output is appended to it (by first seeking to the end-of- file); otherwise, the file is created. <<word The shell input is read up to a line that is the same as word, or to an end-of-file. The resulting document be- comes the standard input. If any character of word is quoted, no interpretation is placed upon the characters of the document; otherwise, parameter and command substi- tution occurs, (unescaped) \newline is ignored, and \ must be used to quote the characters \, $, `, and the first character of word. <&digit Use the file associated with file descriptor digit as standard input. Similarly for the standard output using >&digit. <&- The standard input is closed. Similarly for the standard output using >&-. If any of the above is preceded by a digit, the file descriptor which will be associated with the file is that specified by the digit (in- stead of the default 0 or 1). For example: ... 2>&1 associates file descriptor 2 with the file currently associated with file descriptor 1. The order in which redirections are specified is significant. The shell evaluates redirections left-to-right. For example: ... 1>xxx 2>&1 first associates file descriptor 1 with file then associates file de- scriptor 2 with the same file as descriptor 1, namely while ... 2>&1 1>xxx associates file descriptor 2 with the current value of file descriptor 1 (typically the terminal) and file descriptor 1 with If a command is followed by &, the default standard input for the com- mand is the empty file /dev/null. Otherwise, the environment for the execution of a command contains the file descriptors of the invoking shell as modified by input/output specifications. Environment The environment is a list of strings, conventionally function defini- tions and name-value pairs, that is passed to an executed program in the same way as a normal argument list; see environ(5). The shell in- teracts with the environment in several ways. On invocation, the shell scans the environment and creates a parameter or function for each name found, giving it the corresponding value. If the user modifies the value of any of these parameters or creates new parameters, none of these affects the environment unless the export command is used to bind the shell's parameter to the environment; see also set -a. A parameter may be removed from the environment with the unset command. The envi- ronment seen by any executed command is thus composed of any unmodified name-value pairs originally inherited by the shell, minus any pairs re- moved by unset, plus any modifications or additions, all of which must be noted in export commands. The environment for any simple-command may be augmented by prefixing it with one or more assignments to parameters (but not functions). Thus gets the same environment in both lines below, but the shell has one less variable in the second. (export TERM; TERM=450; tabs) TERM=450 tabs If the -k flag is set, all keyword arguments are placed in the environ- ment, even if they occur after the command name. Signals SIGINT and SIGQUIT (see signal(2)) for an invoked command are ignored if the command is followed by &; otherwise signals have the values in- herited by the shell from its parent (but see also the trap command be- low). Execution Each time a command is executed, the above substitutions are carried out. If the command name matches the name of a defined function, the function is executed in the shell process. (Note how this differs from calling a shell script.) The positional parameters $1, $2, .... are set to the arguments of the function. If the command name does not match a function, but matches one of the builtin commands listed below, it is executed in the shell process. If the command name matches nei- ther a builtin command nor the name of a defined function, a new process is created and an attempt is made to execute the command via exec(2). The shell parameter PATH defines the search path for the directory con- taining the command. Alternative directory names are separated by a colon (:). The default path is :/bin:/usr/bin (specifying the current directory, /bin, and /usr/bin, in that order). Note that the current directory is specified by a null path name, which can appear immedi- ately after the equal sign or between the colon delimiters anywhere else in the path list. If the command name contains a / the search path is not used. Otherwise, each directory in the path is searched for an executable file. If the file has execute permission but is not executable by exec(2), it is assumed to be a `shell script', a file of shell commands. A sub-shell is spawned to read it. A parenthesized command is also executed in a sub-shell. Builtin Commands Input/output redirection is permitted for these commands. File de- scriptor 1 is the default output location. : No effect; the command does nothing. A zero exit code is re- turned. . file Read and execute commands from file and return. The search path specified by PATH is used to find the directory containing file. builtin [ command ] Execute the builtin command (such as break) regardless of func- tions defined with the same name. break [ n ] Exit from the enclosing for or while loop, if any. If n is specified break n levels. continue [ n ] Resume the next iteration of the enclosing for or while loop. If n is specified resume at the n-th enclosing loop. cd [ arg ] Change the current directory to arg. The shell parameter HOME is the default arg. The shell parameter CDPATH defines the search path for the directory containing arg. Alternative di- rectory names are separated by a colon (:). The current direc- tory (default) is specified by a null path name, which can ap- pear immediately after the equal sign or between the colon de- limiters anywhere else in the path list. If arg begins with a the search path is not used. Otherwise, each directory in the path is searched for arg. eval [ arg ... ] The arguments are read as input to the shell and the resulting command(s) executed. exec [ arg ... ] The non-builtin command specified by the arguments is executed in place of this shell without creating a new process. In- put/output arguments may appear and, if no other arguments are given, cause the shell input/output to be modified. exit [ n ] Causes a shell to exit with the exit status specified by n. If n is omitted the exit status is that of the last command exe- cuted (an end-of-file will also cause the shell to exit.) export [ name ... ] The given names are marked for automatic export to the environ- ment of subsequently-executed commands. If no arguments are given, a list of all names that are exported in this shell is printed. read [ name ... ] One line is read from the standard input and the first word is assigned to the first name, the second word to the second name, etc., with leftover words assigned to the last name. The return code is 0 unless an end-of-file is encountered. return [ n ] Causes a function to exit with the return value specified by n. If n is omitted, the return status is that of the last command executed. set [ --aehknptuvx [ arg ... ] ] -a Mark variables which are modified or created for export. -e Exit immediately if a command exits with a non-zero exit status. -f Disable file name generation -k All keyword arguments are placed in the environment for a command, not just those that precede the command name. -n Read commands but do not execute them. -p Remove the definitions for all functions imported from the environment, and set IFS to blank, tab and newline. -t Exit after reading and executing one command. -u Treat unset variables as an error when substituting. -v Print shell input lines as they are read. -x Print commands and their arguments as they are executed. -- Do not change any of the flags; useful in setting $1 to -. Using + rather than - causes these flags to be turned off. These flags can also be used upon invocation of the shell. The current set of flags may be found in $-. The remaining argu- ments are positional parameters and are assigned, in order, to $1, $2, .... If no arguments are given the values of all names are printed. shift [ n ] The positional parameters from $n+1 ... are renamed $1 ... If n is not given, it is assumed to be 1. times Print the accumulated user and system times for processes run from the shell. trap [ arg ] [ n ] ... The command arg is to be read and executed when the shell re- ceives signal(s) n. (Note that arg is scanned once when the trap is set and once when the trap is taken.) Trap commands are executed in order of signal number. Any attempt to set a trap on a signal that was ignored on entry to the current shell is ineffective. If arg is absent all traps n are reset to their original values. If arg is the null string this signal is ig- nored by the shell and by the commands it invokes. If n is 0 the command arg is executed on exit from the shell. The trap command with no arguments prints a list of commands associated with each signal number. umask [ nnn ] The user file-creation mask is set to nnn; see umask(2). If nnn is omitted, the current value of the mask is printed. unset [ name ... ] For each name, remove the corresponding variable or function. The variables PATH, PS1, PS2 and IFS cannot be unset. wait [ n ] Wait for the specified process and report its termination sta- tus. If n is not given all currently active child processes are waited for and the return code is zero. whatis [ name ... ] For each name, print the associated value as a parameter, func- tion, builtin or executable file as appropriate. In each case, the value is printed in a form that would yield the same value if typed as input to the shell itself: parameters are printed as assignments, functions as their definitions, builtins as calls to builtin, and executable files as completed pathnames. Invocation Normally the shell reads commands from the file named in its first ar- gument (standard input default). The remaining arguments are inter- preted as position parameters; see `Parameter substitution' above. If the shell is invoked through exec(2) and the first character of argu- ment zero is -, commands are read first from $HOME/.profile, if it ex- ists. Certain options modify this behavior: -c string Read commands from string; ignore remaining arguments. -s Write shell output (except for builtin commands) on file de- scriptor 2. -i Interactive. Ignore signal SIGTERM (interactive shell is im- mune to kill 0). Catch and ignore SIGINT (wait is interrupt- ible). The shell always ignores SIGQUIT. Other options are described under the set command above. FILES SEE ALSO =(1), echo(1), newgrp(1), test(1), dup(2), exec(2), fork(2), pipe(2), signal(2), umask(2), exit(2), environ(5) B. W. Kernighan and R. Pike, The Unix Programming Environment, Pren- tice-Hall, 1984 DIAGNOSTICS Errors detected by the shell, such as syntax errors, cause the shell to return a non-zero exit status. If the shell is being used non-interac- tively execution of the shell file is abandoned. Otherwise, the shell returns the exit status of the last command executed; see also the exit command. BUGS Errors arising from builtins terminate shell scripts. SH(1)