Bash Index : 'set*' - The 'set*' Bash commands : description, flags and examples

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setcap

Usage

Set file capabilities 
Heard of it in this very interesting article.
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setfacl

Installed with the Debian package

acl

Usage

Set or modify file ACL. This allows granting specific rights to specific users / groups on specific files, without setting global permissions. For instance, if Bob's home directory is : 
drwx------ 64 bob developers 4,0K jan. 15 20:38 bob/
and we would like Alice to have read access to /home/bob/, we can :

Command syntax :

setfacl -m aclEntry someFile
with aclEntry made of prefix:userOrGroup:permissions
  • prefix :
    • u: to change user rights
    • g: to change group rights
    • o: to change other rights. No need to specify userOrGroup
    • d:u: to declare default user rights
    • d:g: to declare default group rights
    • d:o: to declare default other rights
  • permissions : specified either numerically or with symbols (like in chmod) :
    • r : read permission
    • w : write permission
    • x : execute permission
    • X : execute permission, if the file is a directory or already has execute permission for some user (details)
    • - : ignored, rx is equivalent to r-x

Flags

Flag Usage
-b --remove-all
  • remove all extended ACL entries
  • the base ACL entries of the owner, group and others (i.e. Linux permissions) are retained
-d --default All operations apply to the default ACL
-m modify an existing ACL entry
-R Recursive : apply rights to all files and directories. -R must be supplied before -m : -Rm
-x remove an ACL entry :
setfacl -x u:kevin someFile

Example

Grant rw rights to a user on a single file :

setfacl -m u:alice:rw- file

Grant rw rights to a user on all files of a directory :

setfacl -Rm u:alice:rw- directory

Set default rights so that new files will inherit them (details) :

setfacl -m d:u:alice:rw directory/

Grant access to a file tree (source) :

setfacl -R -m user:stuart:rwX path/to/base/directory

If it complains : setfacl: Option -m: Invalid argument near character n :

setfacl -R -m user:httpd:rwx /data
setfacl: Option -m: Invalid argument near character 6
What's wrong ?
  1. Which is the 6th character ? It's the 6th of the value passed to -m : 
    user:httpd:rwx
123456
    Something's wrong with the username
  2. Does this username exist ? None of the commands above return a result : there is no such httpd user.
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set

Usage

Toggle options within a script. At the point in the script where you want the options to take effect, use set -o optionName or, in short form : set -shortOption. These two forms are equivalent.
To disable an option : set +o optionName, or set +shortOption.

Flags

optionName shortOption Usage
noexec n read commands but do not execute them (syntax check)
errexit e abort script at first error : when a command exits with non-zero status, except in these constructs : See notes and examples, Discussion about -e.
nounset u leave script and display an error message when using an unset variable
pipefail
  • By default, pipelines only return a failure if the last command errors.
  • If pipefail is set, the return value of a pipeline is :
    • the value of the last (rightmost) command to exit with a non-zero status
    • or zero if all commands in the pipeline exit successfully
  • When used in combination with -e, pipefail will make a script exit if any command in a pipeline errors.
verbose v print each command to stdout before executing it
xtrace x like verbose, but expands commands
(full list)

Discussion about -e :

-e is often presented as a mandatory flag that should be used in all scripts. But it's also considered by others as a bad / useless practice (stackoverflow.com, mywiki.wooledge.org) because :
  • it's not an error when a test (if, [ ], ) evaluates to false :
    • dealing with a false status is what tests are for
    • treating a false as an error and exiting unconditionally is an over-reaction which brings nothing to the safety of scripts
  • exceptions have been added to -e to workaround the false positives, but these exceptions
    • are not consistent across Bash versions
    • vary whether the process is/is not a subshell
    which actually adds complexity with no gain on safety

Example

Notes and examples with -e (details) :

This doesn't work on :
  • commands within if, until, while block
  • compound commands (list using && or ||)
  • commands with return value being inverted via !
set -e; echo -n 'hello'; true; echo ' world'
Outputs : hello world
echo "set -e; echo -n 'hello'; false; echo ' world'" | bash
Outputs : hello (the echo | bash hack is just to be able to see the result, since because of the false, an exit is executed, forcing to leave the current shell)
set -e; dir='/tmp'; if [ -d "$dir" ] ; then echo "$dir exists"; else echo "$dir does not exist"; fi
Outputs : /tmp exists
No non-success exit code met, -e keeps sleeping.
set -e; dir='/aDirThatDoesNotExist'; if [ -d "$dir" ] ; then echo "$dir exists"; else echo "$dir does not exist"; fi
Outputs : /aDirThatDoesNotExist does not exist
A non-success exit code is met, but -e is muzzled by if.
set -e; dir='/tmp'; [ -d "$dir" ] && echo "$dir exists" || echo "$dir does not exist"
Outputs : /tmp exists
No non-success exit code met, -e keeps sleeping.
set -e; dir='/aDirThatDoesNotExist'; [ -d "$dir" ] && echo "$dir exists" || echo "$dir does not exist"
Outputs : /aDirThatDoesNotExist does not exist
A non-success exit code is met, but -e is muzzled by ????.
#!/usr/bin/env bash
set -e
echo -n 'hello'
true
echo ' world'
Outputs : hello world
#!/usr/bin/env bash
set -e
echo -n 'hello'
false
echo ' world'
Outputs : hello, and returns the exit code 1
#!/usr/bin/env bash
set -e
echo -n 'hello'
if true; then
	echo -n ' wonderful'
fi
echo ' world'
Outputs : hello wonderful world
#!/usr/bin/env bash
set -e
echo -n 'hello'
if false; then
	echo -n ' wonderful'
fi
echo ' world'
Outputs : hello world, and returns the exit code 0.
A non-success exit code is met, but -e is muzzled by if.

Opportunity for a joke :

If you run set -e in a terminal, this will affect the current shell and any further command your "victim" will type. At the 1st non-success return code met (which is VERY easy : try TAB-completing like cd TAB), an exit will be fired, closing the terminal

If you _unintentionally_ run that joke on yourself (), you can disable the -e flag with : set +e

Hack that simulates sending parameters to a script :

  1. set -- whatever
    This construct actually turns whatever into positional parameters. whatever can be :
    • a list of files : file1 file2 file3
    • a shell expansion corresponding to a list of files : /path/to/*jpg
    • the result of a command : $(ls -1)
  2. You can then "read" these as if they were previously sent as parameters :
    set -- $(ls -1); echo $1 $2 $3
    file1 file2 file3		3 files of the current directory
    set -- $(ls -1); echo $@
    ... ... ... ...		all of them