External links
 | The Wikibook Guide to Unix has a page on the topic of: Commands |
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| Developer(s) | AT&T Bell Laboratories |
|---|---|
| Initial release | November 1973 |
| Operating system | Unix and Unix-like |
| Type | Command |
In Unix shells, wait is a command which pauses until execution of a background process has ended.
wait[n]where n is the pid or job ID of a currently executing background process (job). If n is not given, the command waits until all jobs known to the invoking shell have terminated.
wait normally returns the exit status of the last job which terminated. It may also return 127 in the event that n specifies a non-existent job or zero if there were no jobs to wait for.
Because wait needs to be aware of the job table of the current shell execution environment, it is usually implemented as a shell builtin.
This command can be useful where part of a script can execute in parallel to implement a barrier where an upcoming section depends on the successful completion of the preceding sections.
The following example will fetch the src/ directory from a machine named iona using rsync and simultaneously update the libraries on which this program depends, before building the combination.
#!/usr/bin/env bash# Parallel update script which makes use of the wait command# Update local copy rsync iona:src/ . &# Upgrade required libraries, or exit indicating failure if make failed for some reason make -C lib ||exit1# Wait for rsync to terminate (may have already happened) and finish the jobwait make Wait for specified job control id number:
$ ls -R / > /dev/null 2>&1&# start any long running background process[2]1986 $ wait %2 # waits for background job number 2 to terminate, then returns | The Wikibook Guide to Unix has a page on the topic of: Commands |
| File system | |
|---|---|
| Processes | |
| User environment | |
| Text processing | |
| Shell builtins | |
| Searching | |
| Documentation | |
| Software development | |
| Miscellaneous | |
| |
Bash is a Unix shell and command language written by Brian Fox for the GNU Project as a free software replacement for the Bourne shell. First released in 1989, it has been used as the default login shell for most Linux distributions. A version is also available for Windows 10 via the Windows Subsystem for Linux. It is also the default user shell in Solaris 11. Bash was also the default shell in all versions of Apple macOS prior to the 2019 release of macOS Catalina, which changed the default shell to zsh, although Bash remains available as an alternative shell.
A shell script is a computer program designed to be run by the Unix shell, a command-line interpreter. The various dialects of shell scripts are considered to be scripting languages. Typical operations performed by shell scripts include file manipulation, program execution, and printing text. A script which sets up the environment, runs the program, and does any necessary cleanup or logging, is called a wrapper.
In computing, a process is the instance of a computer program that is being executed by one or many threads. It contains the program code and its activity. Depending on the operating system (OS), a process may be made up of multiple threads of execution that execute instructions concurrently.
An environment variable is a dynamic-named value that can affect the way running processes will behave on a computer. They are part of the environment in which a process runs. For example, a running process can query the value of the TEMP environment variable to discover a suitable location to store temporary files, or the HOME or USERPROFILE variable to find the directory structure owned by the user running the process.
In multitasking computer operating systems, a daemon is a computer program that runs as a background process, rather than being under the direct control of an interactive user. Traditionally, the process names of a daemon end with the letter d, for clarification that the process is in fact a daemon, and for differentiation between a daemon and a normal computer program. For example, syslogd is a daemon that implements system logging facility, and sshd is a daemon that serves incoming SSH connections.
The cron command-line utility, also known as cron job, is a job scheduler on Unix-like operating systems. Users who set up and maintain software environments use cron to schedule jobs to run periodically at fixed times, dates, or intervals. It typically automates system maintenance or administration—though its general-purpose nature makes it useful for things like downloading files from the Internet and downloading email at regular intervals.
In computing, time is a command in Unix and Unix-like operating systems. It is used to determine the duration of execution of a particular command.
In computer programming, a return statement causes execution to leave the current subroutine and resume at the point in the code immediately after the instruction which called the subroutine, known as its return address. The return address is saved by the calling routine, today usually on the process's call stack or in a register. Return statements in many languages allow a function to specify a return value to be passed back to the code that called the function.
In computing, kill is a command that is used in several popular operating systems to send signals to running processes.
The exit status of a process in computer programming is a small number passed from a child process to a parent process when it has finished executing a specific procedure or delegated task. In DOS, this may be referred to as an errorlevel.
A command shell is a command-line interface to interact with and manipulate a computer's operating system.
test is a command-line utility found in Unix, Plan 9, and Unix-like operating systems that evaluates conditional expressions. test was turned into a shell builtin command in 1981 with UNIX System III and at the same time made available under the alternate name [.
In computing, a shebang is the character sequence consisting of the characters number sign and exclamation mark at the beginning of a script. It is also called sharp-exclamation, sha-bang, hashbang, pound-bang, or hash-pling.
In computing, exec is a functionality of an operating system that runs an executable file in the context of an already existing process, replacing the previous executable. This act is also referred to as an overlay. It is especially important in Unix-like systems, although it exists elsewhere. As no new process is created, the process identifier (PID) does not change, but the machine code, data, heap, and stack of the process are replaced by those of the new program.
In Unix and Unix-like operating systems, job control refers to control of jobs by a shell, especially interactively, where a "job" is a shell's representation for a process group. Basic job control features are the suspending, resuming, or terminating of all processes in the job/process group; more advanced features can be performed by sending signals to the job. Job control is of particular interest in Unix due to its multiprocessing, and should be distinguished from job control generally, which is frequently applied to sequential execution.
PowerShell is a task automation and configuration management program from Microsoft, consisting of a command-line shell and the associated scripting language. Initially a Windows component only, known as Windows PowerShell, it was made open-source and cross-platform on 18 August 2016 with the introduction of PowerShell Core. The former is built on the .NET Framework, the latter on .NET Core.
A batch file is a script file in DOS, OS/2 and Microsoft Windows. It consists of a series of commands to be executed by the command-line interpreter, stored in a plain text file. A batch file may contain any command the interpreter accepts interactively and use constructs that enable conditional branching and looping within the batch file, such as IF, FOR, and GOTO labels. The term "batch" is from batch processing, meaning "non-interactive execution", though a batch file might not process a batch of multiple data.
A process is a program in execution. An integral part of any modern-day operating system (OS). The OS must allocate resources to processes, enable processes to share and exchange information, protect the resources of each process from other processes and enable synchronization among processes. To meet these requirements, the OS must maintain a data structure for each process, which describes the state and resource ownership of that process, and which enables the OS to exert control over each process.
In a Unix shell, the full stop called the dot command (.) is a command that evaluates commands in a computer file in the current execution context. In C Shell, a similar functionality is provided as the source command, and this name is seen in "extended" POSIX shells as well.
In computing, process substitution is a form of inter-process communication that allows the input or output of a command to appear as a file. The command is substituted in-line, where a file name would normally occur, by the command shell. This allows programs that normally only accept files to directly read from or write to another program.
