Shell tricks

Shell tricks - Programming On Unix

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	josuah Offline \| 23-11-2017, 08:52 AM \| #5

when to quote

More interesting question: when are quotes necessary?

At first I quoted everything, and a few days ago I discovered that quotes are not necessary at many places.

Answer: when commands are interpreted, at "for var in >>here<<".

Commands arguments

If the variable a have the content '1 2 3'

Code:
$ printf '%s\n' $a

1

2

3

As you may know, the shell did cut the arguments from the space in the string. Still the convenient/cumbersome balance of the shell.

Code:
$ printf '%s\n' "$a"

1 2 3

Ok.

Variables assignation do not need quoting, though that does not hurt to add quotes every time.

Code:
$ b=$a/*

$ printf '%s\n' "$b"

1 2 3/*

$ b="$a /*"

$ printf '%s\n' "$b"

1 2 3 /*

$ b=$a /*

/bin/sh: /bin: cannot execute - Is a directory

Yes, we need quotes when we insert a space directly in the variable, as the syntax "MANPAGER=ul man test" is to call the man command with the environment variable MANPAGER set to ul.

The case statements do not need quoting neither, so you can safely do:

Code:
$ a='1 2 3'

$ case $a in

\ 1) echo 1 ;;

\ 2) echo 2 ;;

\ *) echo '*' ;;

\ esac

*

The for statements are special: they need quoting if you want a variable to be considered as a single item:

Code:
$ a='1 2 3'

$ for i in "$a"; do printf '%s\n' "$i"; done

1 2 3

$ for i in  $a;  do printf '%s\n' "$i"; done

1

2

3

The if and while statements

The tests do need quoting. The [ and ] after the if: the /bin/[ file is often a symlink to /bin/test

Code:
$ if [ -z "$a" ]; then echo true; else echo false; fi

false

$ if grep "$a" < $HOME/.profile; then echo true; else echo false; fi

false

$ while [ -z "$a" ]; do echo niet; done

Shell redirection

The <, >, >> operators do not need quoting.

Code:
$ a='1 2 3'

$ echo content > $a

$ cat < *

/bin/sh: cannot open *: No such file or directory

$ cat < $a

Parameters expansion

In the ${var#pattern}, ${var##pattern}, ${var%pattern}, ${var%%pattern} syntax, you have pattern that can contain globs or plain text for removing text from var. The pattern is interpreted just like with double quotes ("):

Code:
$ var='1 2 3'

$ pattern='1 2 '

$ printf '%s\n' "${var#$pattern}"

2

Messages In This Thread

Shell tricks - by josuah - 13-11-2017, 05:27 AM

RE: Shell tricks - by josuah - 13-11-2017, 05:39 AM

RE: Shell tricks - by venam - 14-11-2017, 01:53 AM

RE: Shell tricks - by josuah - 23-11-2017, 07:59 AM

RE: Shell tricks - by josuah - 23-11-2017, 08:52 AM

RE: Shell tricks - by josuah - 04-12-2017, 07:43 AM

RE: Shell tricks - by z3bra - 04-12-2017, 08:11 PM

RE: Shell tricks - by josuah - 05-12-2017, 08:07 AM

RE: Shell tricks - by josuah - 05-12-2017, 09:02 AM

RE: Shell tricks - by josuah - 05-12-2017, 09:08 AM

RE: Shell tricks - by josuah - 08-12-2017, 12:25 PM

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