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| If one executes this simple script, what happens? Bash forks, and the parent waits. The child executes the script, including the {{{chdir(2)}}} system call, and then exits. The parent, which was waiting for the child, harvests the child's exit status (presumably 0 for success), and then bash carries on with the next command. | If one executes this simple script, what happens? Bash forks, resulting in a parent (the interactive shell in which you typed the command) and a child (a new shell that reads and executes the script). The child runs, while the parent waits for it to finish. The child reads and executes the script, changes its current directory to `/tmp`, and then exits. The parent, which was waiting for the child, harvests the child's exit status (presumably 0 for success), and then carries on with the next command. Nowhere in this process has the parent's current working directory changed -- only the child's. |
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| Since the {{{chdir}}} was done by a child process, it has no effect on the parent. | A child process can never affect ''any'' part of the parent's environment, which includes its variables, its current working directory, its open files, its resource limits, etc. |
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| Moreover, there is '''no conceivable way''' you can ''ever'' have a child process affect ''any'' part of the parent's environment, which includes its variables as well as its current working directory. So, how does one go about it? You can still have the {{{cd}}} command in an external file, but you can't ''run it'' as a script. Instead, you must source it with {{{.}}} (or the Bash-only synonym, {{{source}}}). |
So, how does one go about changing the current working directory of the ''parent''? You can still have the {{{cd}}} command in an external file, but you can't ''run it'' as a script. That would cause the forking explained earlier. Instead, you must source it with {{{.}}} (or the Bash-only synonym, {{{source}}}). Sourcing basically means you execute the commands in a file using the ''current'' shell, not in a forked shell (child shell): |
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| echo 'cd /tmp' > $HOME/mycd . $HOME/mycd pwd # Now, we're in /tmp |
echo 'cd /tmp' > "$HOME/mycd" # Create a file that contains the 'cd /tmp' command. . $HOME/mycd # Source that file, executing the 'cd /tmp' command in the current shell. pwd # Now, we're in /tmp |
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| The same thing applies to setting variables. {{{.}}} ('dot') the file that contains the commands; don't try to run it. | The same thing applies to setting variables. {{{.}}} ("dot in") the file that contains the commands; don't try to run it. |
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| Functions are run in the same same shell, so it is possible to put |
If the command you execute is a function, not a script, it will be executed in the current shell. Therefore, it's possible to define a function to do what we tried to do with an external file in the examples above, without needing to "dot in" or "source" anything. Define the following function and then call it simply by typing `mycd`: |
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| in .bashrc or similar, and then use {{{mycd}}} to change the directory. | Put it in `~/.bashrc` [[DotFiles|or similar]] if you want the function to be available automatically in every new shell you open. Some people prefer to use [[BashFAQ/080|aliases]] instead of functions. Functions are more powerful, more general, more flexible, and... some people just don't seem to like them. {{{ alias mycd='cd /tmp' # Equivalent to the function shown above. }}} {{{ alias cdlstmp='cd /tmp && ls tmp*' # will take you to /tmp and show you what files there are starting with "tmp" }}} {{{ cdls() { cd "$1" && ls; } # cannot be done with an alias # usage 'cdls directory' }}} |
I'm trying to write a script that will change directory (or set a variable), but after the script finishes, I'm back where I started (or my variable isn't set)!
Consider this:
#!/bin/sh cd /tmp
If one executes this simple script, what happens? Bash forks, resulting in a parent (the interactive shell in which you typed the command) and a child (a new shell that reads and executes the script). The child runs, while the parent waits for it to finish. The child reads and executes the script, changes its current directory to /tmp, and then exits. The parent, which was waiting for the child, harvests the child's exit status (presumably 0 for success), and then carries on with the next command. Nowhere in this process has the parent's current working directory changed -- only the child's.
A child process can never affect any part of the parent's environment, which includes its variables, its current working directory, its open files, its resource limits, etc.
So, how does one go about changing the current working directory of the parent? You can still have the cd command in an external file, but you can't run it as a script. That would cause the forking explained earlier. Instead, you must source it with . (or the Bash-only synonym, source). Sourcing basically means you execute the commands in a file using the current shell, not in a forked shell (child shell):
echo 'cd /tmp' > "$HOME/mycd" # Create a file that contains the 'cd /tmp' command. . $HOME/mycd # Source that file, executing the 'cd /tmp' command in the current shell. pwd # Now, we're in /tmp
The same thing applies to setting variables. . ("dot in") the file that contains the commands; don't try to run it.
If the command you execute is a function, not a script, it will be executed in the current shell. Therefore, it's possible to define a function to do what we tried to do with an external file in the examples above, without needing to "dot in" or "source" anything. Define the following function and then call it simply by typing mycd:
mycd() { cd /tmp; }Put it in ~/.bashrc or similar if you want the function to be available automatically in every new shell you open.
Some people prefer to use aliases instead of functions. Functions are more powerful, more general, more flexible, and... some people just don't seem to like them.
alias mycd='cd /tmp' # Equivalent to the function shown above.
alias cdlstmp='cd /tmp && ls tmp*' # will take you to /tmp and show you what files there are starting with "tmp"
cdls() { cd "$1" && ls; }
# cannot be done with an alias
# usage 'cdls directory'