Secuencias de comandos de shell: opciones -z y -n con if

Puede encontrar una referencia muy buena para los operadores de bash aquí . Si estás usando un shell diferente, simplemente busca <my shell> operators y encontrarás todo lo que necesitas. En tu caso particular, estás usando:

-n string is not null. -z string is null, that is, has zero length 

Para ilustrar:

$ foo="bar"; $ [ -n "$foo" ] && echo "foo is not null" foo is not null $ [ -z "$foo" ] && echo "foo is null" $ foo=""; $ [ -n "$foo" ] && echo "foo is not null" $ [ -z "$foo" ] && echo "foo is null" foo is null 

Comentarios

• Si tiene problemas para lograr que -n funcione, podría deberse a que está siguiendo algunas guías incorrectas en la web (por ejemplo, GeeksforGeeks o TutorialsPoint ) que no citan las variables. Esta respuesta, y la guía vinculada aquí, citan correctamente Si usa -n sin citar, le dirá que ' no está vacío incluso cuando lo está. @terdon, ¡muchas gracias!

Para extender la respuesta de terdon , encontré que Unix / Linux – Operadores básicos de Shell en Tutorials Point también incluye operadores relacionados con archivos (así como otros útiles).

-b file Checks if file is a block special file; if yes, then the condition becomes true. [ -b $file ] is false. -c file Checks if file is a character special file; if yes, then the condition becomes true. [ -c $file ] is false. -d file Checks if file is a directory; if yes, then the condition becomes true. [ -d $file ] is not true. -f file Checks if file is an ordinary file as opposed to a directory or special file; if yes, then the condition becomes true. [ -f $file ] is true. -g file Checks if file has its set group ID (SGID) bit set; if yes, then the condition becomes true. [ -g $file ] is false. -k file Checks if file has its sticky bit set; if yes, then the condition becomes true. [ -k $file ] is false. -p file Checks if file is a named pipe; if yes, then the condition becomes true. [ -p $file ] is false. -t file Checks if file descriptor is open and associated with a terminal; if yes, then the condition becomes true. [ -t $file ] is false. -u file Checks if file has its Set User ID (SUID) bit set; if yes, then the condition becomes true. [ -u $file ] is false. -r file Checks if file is readable; if yes, then the condition becomes true. [ -r $file ] is true. -w file Checks if file is writable; if yes, then the condition becomes true. [ -w $file ] is true. -x file Checks if file is executable; if yes, then the condition becomes true. [ -x $file ] is true. -s file Checks if file has size greater than 0; if yes, then condition becomes true. [ -s $file ] is true. -e file Checks if file exists; is true even if file is a directory but exists. [ -e $file ] is true.