What does "chmod +x <filename>" do and how do I use it?

Question

I want to write the Ubuntu analogue of a "batch file" (a shell script). But I don't know how to use chmod +x filename command to make it so that the script can be run. Nor do I know where to use it.

Answer 1

In Short:

chmod +x on a file (your script) only means, that you'll make it executable. Right click on your script and chose Properties -> Permissions -> Allow executing file as program, leaves you with the exact same result as the command in terminal.

If a file you want to change permissions on is located within the systems directory you may need to be root, like so: (be careful, while using sudo command)

sudo chmod +x /usr/share/testfolder/aFile 

Also it is not clear, what exactly you want to archive here. Please edit your question and give some more detail on the actual problem!

You can also refer to this question, for more information: chmod u+x' versus 'chmod +x

In Long:

Type man chmod in a terminal window (Ctrl+Alt+T) and you'll get the following output:

---

NAME: chmod - change file mode bits

SYNOPSIS

chmod [OPTION]... MODE[,MODE]... FILE...
chmod [OPTION]... OCTAL-MODE FILE...
chmod [OPTION]... --reference=RFILE FILE...

DESCRIPTION

This  manual page documents the GNU version of chmod.  chmod changes the
file mode bits of each given file according to mode, which can be either
a  symbolic representation of changes to make, or an octal number repre‐
senting the bit pattern for the new mode bits.

The format of a symbolic mode  is  [ugoa...][[+-=][perms...]...],  where
perms  is  either  zero or more letters from the set rwxXst, or a single
letter from the set ugo.  Multiple symbolic modes can  be  given,  sepa‐
rated by commas.

A  combination  of  the letters ugoa controls which users' access to the
file will be changed: the user who owns  it  (u),  other  users  in  the
file's  group (g), other users not in the file's group (o), or all users
(a).  If none of these are given, the effect is as if a were given,  but
bits that are set in the umask are not affected.

The  operator  +  causes  the selected file mode bits to be added to the
existing file mode bits of each file; - causes them to be removed; and =
causes them to be added and causes unmentioned bits to be removed except
that a directory's unmentioned set  user  and  group  ID  bits  are  not
affected.

The  letters  rwxXst  select file mode bits for the affected users: read
(r), write (w), execute (or search for directories) (x),  execute/search
only  if  the  file is a directory or already has execute permission for
some user (X), set user or group ID on execution (s),  restricted  dele‐
tion  flag  or sticky bit (t).  Instead of one or more of these letters,
you can specify exactly one of the letters ugo: the permissions  granted
to  the  user  who  owns  the file (u), the permissions granted to other
users who are members of the  file's  group  (g),  and  the  permissions
granted  to  users  that  are in neither of the two preceding categories
(o).

A numeric mode is from one to four octal digits (0-7), derived by adding
up  the  bits with values 4, 2, and 1.  Omitted digits are assumed to be
leading zeros.  The first digit selects the set  user  ID  (4)  and  set
group ID (2) and restricted deletion or sticky (1) attributes.  The sec‐
ond digit selects permissions for the user who owns the file: read  (4),
write  (2),  and  execute  (1);  the third selects permissions for other
users in the file's group, with the same  values;  and  the  fourth  for
other users not in the file's group, with the same values.

chmod  never changes the permissions of symbolic links; the chmod system
call cannot change their permissions.  This is not a problem  since  the
permissions  of  symbolic  links are never used.  However, for each sym‐
bolic link listed on the command line, chmod changes the permissions  of
the  pointed-to file.  In contrast, chmod ignores symbolic links encoun‐
tered during recursive directory traversals.

SETUID AND SETGID BITS

chmod clears the set-group-ID bit of a regular file if the file's  group
ID  does  not  match  the user's effective group ID or one of the user's
supplementary group IDs, unless the  user  has  appropriate  privileges.
Additional  restrictions may cause the set-user-ID and set-group-ID bits
of MODE or RFILE to be ignored.  This behavior depends on the policy and
functionality of the underlying chmod system call.  When in doubt, check
the underlying system behavior.

OPTIONS

Change the mode of each FILE to MODE.

   -c, --changes
          like verbose but report only when a change is made

   --no-preserve-root
          do not treat `/' specially (the default)

   --preserve-root
          fail to operate recursively on `/'

   -f, --silent, --quiet
          suppress most error messages

   -v, --verbose
          output a diagnostic for every file processed

   --reference=RFILE
          use RFILE's mode instead of MODE values

   -R, --recursive
          change files and directories recursively

   --help display this help and exit

   --version
          output version information and exit

   Each MODE is of the form `[ugoa]*([-+=]([rwxXst]*|[ugo]))+'.

Answer 2

A batch file and a shell script are two terms meaning effectively the same under Linux. The term script is far more often used, though.

The most simple shell script file just contains commands as you'd type them on the command line (that is, the Bash command interpreter). In theory you can even replace the interpreter with any language you like (and have an interpreter for). To be more explicit, it's suggested you start your first line with

#!/bin/sh (if you wan't maximum portability with legacy systems)

or

#!/bin/bash (if you want some extra features, you probably don't care about today)

After this line enter your commands, one on each line. There are lots of extra constructs beyond the scope of this questions, see man bash or http://www.tldp.org/LDP/Bash-Beginners-Guide/Bash-Beginners-Guide.pdf (for Beginners) or http://www.tldp.org/LDP/abs/abs-guide.pdf (for more advanced questions).

To actually run your script, there are two requirements: First, the interpreter process needs to read the file, and second it checks if it is marked as executable. For convenience reasons it's also useful to be able to write to your script (so you can make changes or fixes should need be).

Assuming further that you want team members and other also to be able to run (and see) your script, but that you don't want them to manipulate it, the combination of

• execute rights for all (a+x or +x, as a is the default),
• read rights for all (a+r or +r, as a is again the default),
• write permissions just for you (u=w)

are usually sensible values of your file permissions. You can type the single actions concatenated, separated with a comma.

While this "action language" is very fascinating (note the difference of the + and the = operator which result in different outcomes depending of the permission setting before you changed them), they are tedious to type.

As all of the actions create bit masks that are applied internally, you can type the bitmasks (see man chmod for details) also directly.

For a shellscript chmod 755 myscript.sh makes most sense in at least 95% of all cases.

Answer 3

First, your script must declare which interpreter to use. You do this in the first line of the file. If it's a shell script, it should be #!/bin/sh or #!/bin/bash.

So here's a script that writes your username: echo-whoami.sh

#!/bin/sh
echo $(whoami)

To make it executable, use:

chmod +x echo-whoami.sh

Then you can run it using:

./echo-whoami.sh